Semiconductor device

ABSTRACT

In a semiconductor device using a transistor including an oxide semiconductor, a change in electrical characteristics is suppressed and reliability is improved. The semiconductor device includes a gate electrode over an insulating surface; an oxide semiconductor film overlapping with the gate electrode; a gate insulating film that is between the gate electrode and the oxide semiconductor film and in contact with the oxide semiconductor film; a protective film in contact with a surface of the oxide semiconductor film that is an opposite side of a surface in contact with the gate insulating film; and a pair of electrodes in contact with the oxide semiconductor film. The spin density of the gate insulating film or the protective film measured by electron spin resonance spectroscopy is lower than 1×10 18  spins/cm 3 , preferably higher than or equal to 1×10 17  spins/cm 3  and lower than 1×10 18  spins/cm 3 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. In addition, the present invention relates to a process, amachine, manufacture, or a composition of matter. In particular, thepresent invention relates to a semiconductor device, a display device, alight-emitting device, a power storage device, a driving method thereof,or a manufacturing method thereof. Furthermore in particular, thepresent invention relates to a semiconductor device including afield-effect transistor.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A semiconductor element such as a transistor, asemiconductor circuit, an arithmetic device, and a memory device areeach one embodiment of a semiconductor device. An imaging device, adisplay device, a liquid crystal display device, a light-emittingdevice, an electro-optical device, a power generation device (includinga thin film solar cell, an organic thin film solar cell, and the like),and an electronic device may each include a semiconductor device.

2. Description of the Related Art

Transistors used for most flat panel displays typified by a liquidcrystal display device and a light-emitting display device are formedusing silicon semiconductors such as amorphous silicon, single crystalsilicon, and polycrystalline silicon provided over glass substrates.Further, such a transistor employing such a silicon semiconductor isused in integrated circuits (ICs) and the like.

In recent years, attention has been drawn to a technique in which,instead of a silicon semiconductor, a metal oxide exhibitingsemiconductor characteristics is used in transistors. Note that in thisspecification, a metal oxide exhibiting semiconductor characteristics isreferred to as an oxide semiconductor.

For example, a technique is disclosed in which a transistor ismanufactured using zinc oxide or an In—Ga—Zn-based oxide as an oxidesemiconductor and the transistor is used as a switching element or thelike of a pixel of a display device (see Patent Documents 1 and 2).

It has been pointed out that hydrogen is a supply source of carriersparticularly in an oxide semiconductor. Therefore, some measures need tobe taken to prevent hydrogen from entering the oxide semiconductor atthe time of forming the oxide semiconductor. Further, variation in athreshold voltage is suppressed by reducing the amount of hydrogencontained in the oxide semiconductor film or a gate insulating film incontact with the oxide semiconductor (see Patent Document 3).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055-   [Patent Document 3] Japanese Published Patent Application No.    2009-224479

SUMMARY OF THE INVENTION

However, nitrogen becomes a source for supplying carriers in a similarmanner to hydrogen. Thus, when a large amount of nitrogen is containedin a film in contact with an oxide semiconductor film, the electricalcharacteristics, typically the threshold voltage, of a transistorincluding the oxide semiconductor film is changed. Further, there is aproblem in that electrical characteristics vary among the transistors.

It is one object of one embodiment of the present invention to suppressa change in electrical characteristics and to improve reliability in asemiconductor device using a transistor including an oxidesemiconductor. It is another object of one embodiment of the presentinvention to provide a semiconductor device with low power consumption.A yet still further object of one embodiment of the present invention isto provide a novel semiconductor device.

One embodiment of the present invention is a semiconductor deviceincluding a gate electrode over an insulating surface; an oxidesemiconductor film overlapping with the gate electrode; a gateinsulating film that is between the gate electrode and the oxidesemiconductor film and in contact with the oxide semiconductor film; aprotective film in contact with a surface of the oxide semiconductorfilm that is an opposite side of a surface in contact with the gateinsulating film; and a pair of electrodes in contact with the oxidesemiconductor film. The spin density of the gate insulating film or theprotective film measured by electron spin resonance (ESR) spectroscopyis lower than 1×10¹⁸ spins/cm³, preferably higher than or equal to1×10¹⁷ spins/cm³ and lower than 1×10¹⁸ spins/cm³.

In an electron spin resonance spectrum of the gate insulating film orthe protective film, a first signal that appears at a g-factor ofgreater than or equal to 2.037 and smaller than or equal to 2.039, asecond signal that appears at a g-factor of greater than or equal to2.001 and smaller than or equal to 2.003, and a third signal thatappears at a g-factor of greater than or equal to 1.964 and smaller thanor equal to 1.966 are observed. The split width of the first and secondsignals and the split width of the second and third signals that areobtained by measurement using an X-band are each approximately 5 mT.

In the electron spin resonance spectrum of the gate insulating film orthe protective film, a signal attributed to nitrogen oxide is observed.The nitrogen oxide contains nitrogen monoxide or nitrogen dioxide.

The protective film, the oxide semiconductor film, and the gateinsulating film may be provided between the insulating surface and thegate electrode. Alternatively, the gate electrode and the gateinsulating film may be provided between the insulating surface and theoxide semiconductor film.

The pair of electrodes may be provided between the oxide semiconductorfilm and the protective film. Alternatively, the pair of electrodes maybe provided between the oxide semiconductor film and the gate insulatingfilm.

With one embodiment of the present invention, a change in the electricalcharacteristics of a transistor including an oxide semiconductor film issuppressed and reliability can be improved. Further, according to oneembodiment of the present invention, a semiconductor device with lesspower consumption can be provided. According to one embodiment of thepresent invention, a novel semiconductor device can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views illustrating oneembodiment of a transistor.

FIGS. 2A to 2D are cross-sectional views illustrating one embodiment ofa method for manufacturing a transistor.

FIGS. 3A and 3B are each a cross-sectional view illustrating oneembodiment of a transistor.

FIGS. 4A and 4B are each a cross-sectional view illustrating oneembodiment of a transistor.

FIGS. 5A to 5C are a top view and cross-sectional views illustrating oneembodiment of a transistor.

FIGS. 6A to 6D are a top view and cross-sectional views illustrating oneembodiment of a transistor.

FIGS. 7A to 7C are each a diagram showing a band structure of atransistor.

FIG. 8 shows a crystalline model of c-SiO₂.

FIG. 9 shows a model in which a nitrogen dioxide molecule is introducedinto an interstitial site of a c-SiO₂ model.

FIG. 10 shows a model in which a dinitrogen monoxide molecule isintroduced into an interstitial site of a c-SiO₂ model.

FIG. 11 shows a model in which a nitrogen monoxide molecule isintroduced into an interstitial site of a c-SiO₂ model.

FIG. 12 shows a model in which a nitrogen atom is introduced into aninterstitial site of a c-SiO₂ model.

FIG. 13 is a band diagram.

FIGS. 14A and 14B each show a model of a cluster structure.

FIG. 15 illustrates a mechanism of a phenomenon in which the thresholdvoltage of a transistor is shifted in the positive direction.

FIGS. 16A to 16D illustrate bulk models.

FIGS. 17A and 17B illustrate the relation between formation energy andtransition levels and electron configurations of defects.

FIG. 18 illustrates a change in the Fermi level and a change in thecharge states of defects.

FIG. 19 illustrates the structure of a model.

FIGS. 20A and 20B illustrate the relation between the formation energyand the transition levels of V_(o)H and the thermodynamic transitionlevel of V_(o)H.

FIG. 21 shows the relation between the carrier density and the defectdensity of V_(o)H.

FIG. 22 illustrates a band structure of DOS inside an oxidesemiconductor film and in the vicinity of the interface of the oxidesemiconductor film.

FIG. 23 is a graph showing deterioration of a transistor including anoxide semiconductor film in a dark state.

FIG. 24 illustrates deterioration of a transistor including an oxidesemiconductor film in a dark state.

FIG. 25 is a graph showing deterioration of a transistor including anoxide semiconductor film under light irradiation.

FIG. 26 illustrates showing deterioration of a transistor including anoxide semiconductor film under light irradiation.

FIG. 27 is a graph showing deterioration of a transistor including anoxide semiconductor film under light irradiation.

FIGS. 28A to 28F illustrate a model where an oxide semiconductor film ishighly purified to be intrinsic.

FIGS. 29A to 29C illustrate a crystalline model of InGaZnO₄ and adefect.

FIGS. 30A and 30B illustrate a structure of a model in which a carbonatom is put in (6) and its density of states.

FIGS. 31A and 31B illustrate a structure of a model in which an indiumatom is replaced with a carbon atom and its density of states.

FIGS. 32A and 32B illustrate a structure of a model in which a galliumatom is replaced with a carbon atom and its density of states.

FIGS. 33A and 33B illustrate a structure of a model in which a zinc atomis replaced with a carbon atom and its density of states.

FIGS. 34A to 34C are a top view and cross-sectional views illustratingone embodiment of a transistor.

FIGS. 35A to 35D are cross-sectional views illustrating one embodimentof a method for manufacturing a transistor.

FIGS. 36A and 36B are each a cross-sectional view of one embodiment of atransistor.

FIGS. 37A to 37C are a top view and cross-sectional views illustratingone embodiment of a transistor.

FIGS. 38A to 38C illustrate a structure of a display panel of oneembodiment.

FIG. 39 illustrates a display module.

FIGS. 40A to 40D are each an external view of an electronic deviceaccording to one embodiment.

FIGS. 41A to 41C are graphs of results of TDS analysis.

FIG. 42 shows results of TDS analysis.

FIG. 43 shows results of TDS analysis.

FIGS. 44A and 44B show results of SIMS analysis.

FIGS. 45A to 45C show ESR measurement results.

FIGS. 46A to 46C show ESR measurement results.

FIG. 47 shows Vg-Id characteristics of a transistor.

FIG. 48 shows the amount of change in the threshold voltage and theamount of change in the shift value of transistors after gate BT stresstests and after gate BT photostress tests.

FIG. 49 shows Vg-Id characteristics of a transistor.

FIG. 50 shows the amount of change in the threshold voltage and theamount of change in the shift value of transistors after gate BT stresstests and after gate BT photostress tests.

FIG. 51 shows the amount of change in spin density and the amount ofchange in threshold voltage.

FIGS. 52A and 52B show results of SIMS analysis.

FIGS. 53A and 53B show ESR measurement results.

FIGS. 54A and 54B show results of TDS analysis.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention are described below in detail withreference to the drawings. Note that the present invention is notlimited to the following description, and it is easily understood bythose skilled in the art that the mode and details can be variouslychanged without departing from the spirit and scope of the presentinvention. Therefore, the present invention should not be construed asbeing limited to the description in the following embodiments andexamples. In addition, in the following embodiments and examples, thesame portions or portions having similar functions are denoted by thesame reference numerals or the same hatching patterns in differentdrawings, and description thereof is not repeated.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such a scale.

In addition, terms such as “first”, “second”, and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically. Therefore, forexample, the term “first” can be replaced with the term “second”,“third”, or the like as appropriate.

Functions of a “source” and a “drain” are sometimes replaced with eachother when the direction of current flow is changed in circuitoperation, for example. Therefore, the terms “source” and “drain” can beused to denote the drain and the source, respectively, in thisspecification.

Note that a voltage refers to a difference between potentials of twopoints, and a potential refers to electrostatic energy (electricpotential energy) of a unit charge at a given point in an electrostaticfield. Note that in general, a difference between a potential of onepoint and a reference potential (e.g., a ground potential) is merelycalled a potential or a voltage, and a potential and a voltage are usedas synonymous words in many cases. Thus, in this specification, apotential may be rephrased as a voltage and a voltage may be rephrasedas a potential unless otherwise specified.

Note that a transistor including an oxide semiconductor film is ann-channel transistor; therefore, in this specification, a transistorthat can be regarded as having no drain current flowing therein when agate voltage is 0 V is defined as a transistor having normally-offcharacteristics. In contrast, a transistor that can be regarded ashaving a drain current flowing therein when the gate voltage is 0 V isdefined as a transistor having normally-on characteristics.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of thepresent invention and a method for manufacturing the semiconductordevice are described with reference to drawings. A transistor 10described in this embodiment has a bottom-gate structure.

<1. Structure of Transistor>

FIGS. 1A to 1C are a top view and cross-sectional views of thetransistor 10 included in a semiconductor device. FIG. 1A is a top viewof the transistor 10, FIG. 1B is a cross-sectional view taken alongdashed-dotted line A-B in FIG. 1A, and FIG. 1C is a cross-sectional viewtaken along dashed-dotted line C-D in FIG. 1A. Note that in FIG. 1A, asubstrate 11, a gate insulating film 15, a protective film 21, and thelike are omitted for simplicity.

The transistor 10 illustrated in FIGS. 1A to 1C includes a gateelectrode 13 over the substrate 11, the gate insulating film 15 over thesubstrate 11 and the gate electrode 13, an oxide semiconductor film 17overlapping with the gate electrode 13 with the gate insulating film 15therebetween, and a pair of electrodes 19 and 20 in contact with theoxide semiconductor film 17. The protective film 21 is formed over thegate insulating film 15, the oxide semiconductor film 17, and the pairof electrodes 19 and 20.

The protective film 21 is in contact with a surface of the oxidesemiconductor film 17 that is an opposite side of a surface in contactwith the gate insulating film 15. In other words, the protective film 21has a function of protecting a region (hereinafter referred to as a backchannel region) of the oxide semiconductor film 17 that is on theopposite side of a region where a channel is formed.

In this embodiment, a film in contact with the oxide semiconductor film17, typically, at least one of the gate insulating film 15 and theprotective film 21 is an oxide insulating film containing nitrogen andhaving a small number of defects.

Typical examples of the oxide insulating film containing nitrogen andhaving a small number of defects include a silicon oxynitride film andan aluminum oxynitride film. Note that a “silicon oxynitride film” or an“aluminum oxynitride film” refers to a film that contains more oxygenthan nitrogen, and a “silicon nitride oxide film” or an “aluminumnitride oxide film” refers to a film that contains more nitrogen thanoxygen.

In an ESR spectrum at 100 K or lower of the oxide insulating film with asmall number of defects, a first signal that appears at a g-factor ofgreater than or equal to 2.037 and smaller than or equal to 2.039, asecond signal that appears at a g-factor of greater than or equal to2.001 and smaller than or equal to 2.003, and a third signal thatappears at a g-factor of greater than or equal to 1.964 and smaller thanor equal to 1.966 are observed. The split width of the first and secondsignals and the split width of the second and third signals that areobtained by ESR measurement using an X-band are each approximately 5 mT.The sum of the spin densities of the first signal that appears at ag-factor of greater than or equal to 2.037 and smaller than or equal to2.039, the second signal that appears at a g-factor of greater than orequal to 2.001 and smaller than or equal to 2.003, and the third signalthat appears at a g-factor of greater than or equal to 1.964 and smallerthan or equal to 1.966 is lower than 1×10¹⁸ spins/cm³, typically higherthan or equal to 1×10¹⁷ spins/cm³ and lower than 1×10¹⁸ spins/cm³.

In the ESR spectrum at 100 K or lower, the first signal that appears ata g-factor of greater than or equal to 2.037 and smaller than or equalto 2.039, the second signal that appears at a g-factor of greater thanor equal to 2.001 and smaller than or equal to 2.003, and the thirdsignal that appears at a g-factor of greater than or equal to 1.964 andsmaller than or equal to 1.966 correspond to signals attributed tonitrogen oxide (NO_(x); x is greater than or equal to 0 and smaller thanor equal to 2, preferably greater than or equal to 1 and smaller than orequal to 2). Typical examples of nitrogen oxide include nitrogenmonoxide and nitrogen dioxide. In other words, when the spin densitiesof signals that appear at a g-factor of greater than or equal to 1.964and smaller than or equal to 1.966 to a g-factor of greater than orequal to 2.037 and smaller than or equal to 2.039 are lower, thenitrogen oxide content in an oxide insulating film is lower.

When at least one of the gate insulating film 15 and the protective film21 in contact with the oxide semiconductor film 17 contains a smallamount of nitrogen oxide as described above, the carrier trap at theinterface between the oxide semiconductor film 17 and the gateinsulating film 15 or the interface between the oxide semiconductor film17 and the protective film 21 can be inhibited. As a result, a change inthe threshold voltage of the transistor included in the semiconductordevice can be reduced, which leads to a reduced change in the electricalcharacteristics of the transistor.

At least one of the gate insulating film 15 and the protective film 21preferably has a nitrogen concentration measured by secondary ion massspectrometry (SIMS) of lower than or equal to 6×10²⁰ atoms/cm³. In thatcase, nitrogen oxide is unlikely to be generated in at least one of thegate insulating film 15 and the protective film 21, so that the carriertrap at the interface between the oxide semiconductor film 17 and thegate insulating film 15 or the interface between the oxide semiconductorfilm 17 and the protective film 21 can be inhibited. Furthermore, achange in the threshold voltage of the transistor included in thesemiconductor device can be reduced, which leads to a reduced change inthe electrical characteristics of the transistor.

The details of other components of the transistor 10 are describedbelow.

There is no particular limitation on the property of a material and thelike of the substrate 11 as long as the material has heat resistanceenough to withstand at least later heat treatment. For example, a glasssubstrate, a ceramic substrate, a quartz substrate, or a sapphiresubstrate may be used as the substrate 11. Alternatively, a singlecrystal semiconductor substrate or a polycrystalline semiconductorsubstrate made of silicon, silicon carbide, or the like, a compoundsemiconductor substrate made of silicon germanium or the like, a siliconon insulator (SOI) substrate, or the like may be used as the substrate11. Furthermore, any of these substrates further provided with asemiconductor element may be used as the substrate 11.

Alternatively, a flexible substrate may be used as the substrate 11, andthe transistor 10 may be provided directly on the flexible substrate.Further alternatively, a separation layer may be provided between thesubstrate 11 and the transistor 10. The separation layer can be usedwhen part or the whole of a semiconductor device formed over theseparation layer is separated from the substrate 11 and transferred ontoanother substrate. In such a case, the transistor 10 can be transferredto a substrate having low heat resistance or a flexible substrate aswell.

A base insulating film may be provided between the substrate 11 and thegate electrode 13. Examples of the base insulating film include asilicon oxide film, a silicon oxynitride film, a silicon nitride film, asilicon nitride oxide film, a gallium oxide film, a hafnium oxide film,an yttrium oxide film, an aluminum oxide film, and an aluminumoxynitride film. Note that when silicon nitride, gallium oxide, hafniumoxide, yttrium oxide, aluminum oxide, or the like is used for the baseinsulating film, it is possible to suppress diffusion of impurities suchas alkali metal, water, and hydrogen from the substrate 11 into theoxide semiconductor film 17.

The gate electrode 13 can be formed using a metal element selected fromaluminum, chromium, copper, tantalum, titanium, molybdenum, andtungsten; an alloy containing any of these metal elements as acomponent; an alloy containing these metal elements in combination; orthe like. Further, one or more metal elements selected from manganeseand zirconium may be used. The gate electrode 13 may have a single-layerstructure or a layered structure of two or more layers. For example, asingle-layer structure of an aluminum film containing silicon, atwo-layer structure in which a titanium film is stacked over an aluminumfilm, a two-layer structure in which a titanium film is stacked over atitanium nitride film, a two-layer structure in which a tungsten film isstacked over a titanium nitride film, a two-layer structure in which atungsten film is stacked over a tantalum nitride film or a tungstennitride film, a three-layer structure in which a titanium film, analuminum film, and a titanium film are stacked in this order, and thelike can be given. Alternatively, a film, an alloy film, or a nitridefilm that contains aluminum and one or more elements selected fromtitanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium may be used.

The gate electrode 13 can also be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide containing silicon oxide.It is also possible to have a layered structure formed using the abovelight-transmitting conductive material and the above metal element.

In the case where the protective film 21 is formed using an oxideinsulating film containing nitrogen and having a small number ofdefects, the gate insulating film 15 can be formed to have asingle-layer structure or a stacked-layer structure using, for example,any of silicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, aluminum oxide, hafnium oxide, gallium oxide, Ga—Zn-based metaloxide, and the like. Note that an oxide insulating film is preferablyused for at least a region of the gate insulating film 15, which is incontact with the oxide semiconductor film 17, in order to improvecharacteristics of the interface with the oxide semiconductor film 17.

Further, it is possible to prevent outward diffusion of oxygen from theoxide semiconductor film 17 and entry of hydrogen, water, or the likeinto the oxide semiconductor film 17 from the outside by providing aninsulating film having a blocking effect against oxygen, hydrogen,water, and the like for the gate insulating film 15. As the insulatingfilm having a blocking effect against oxygen, hydrogen, water, and thelike, an aluminum oxide film, an aluminum oxynitride film, a galliumoxide film, a gallium oxynitride film, an yttrium oxide film, an yttriumoxynitride film, a hafnium oxide film, a hafnium oxynitride film, and asilicon nitride film can be given as examples.

The gate insulating film 15 may be formed using a high-k material suchas hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current of the transistor can be reduced.

The thickness of the gate insulating film 15 is greater than or equal to5 nm and less than or equal to 400 nm, preferably greater than or equalto 10 nm and less than or equal to 300 nm, more preferably greater thanor equal to 50 nm and less than or equal to 250 nm.

The oxide semiconductor film 17 is formed using a metal oxide filmcontaining at least In or Zn; as a typical example, an In—Ga oxide film,an In—Zn oxide film, or an In-M-Zn oxide film (M is Al, Ga, Y, Zr, La,Ce, or Nd) can be given.

Note that in the case where the oxide semiconductor film 17 contains anIn-M-Zn oxide, the proportion of in and the proportion of M, not takingZn and O into consideration, are greater than 25 atomic % and less than75 atomic %, respectively, preferably greater than 34 atomic % and lessthan 66 atomic %, respectively.

The energy gap of the oxide semiconductor film 17 is 2 eV or more,preferably 2.5 eV or more, further preferably 3 eV or more. With the useof an oxide semiconductor having such a wide energy gap, the off-statecurrent of the transistor 10 can be reduced.

The thickness of the oxide semiconductor film 17 is greater than orequal to 3 nm and less than or equal to 200 nm, preferably greater thanor equal to 3 nm and less than or equal to 100 nm, further preferablygreater than or equal to 3 nm and less than or equal to 50 nm.

In the case where the oxide semiconductor film 17 contains an In-M-Znoxide (M represents Al, Ga, Y, Zr, La, Ce, or Nd), it is preferable thatthe atomic ratio of metal elements of a sputtering target used forforming a film of the In-M-Zn oxide satisfy In M and Zn M. As the atomicratio of metal elements of such a sputtering target, In:M:Zn=1:1:1,In:M:Zn=1:1:1.2, and In:M:Zn=3:1:2 are preferable. Note that the atomicratios of metal elements in the formed oxide semiconductor film 17 varyfrom the above atomic ratio of metal elements of the sputtering targetwithin a range of ±40% as an error.

Hydrogen contained in the oxide semiconductor reacts with oxygen bondedto a metal atom to be water, and also causes oxygen vacancies in alattice from which oxygen is released (or a portion from which oxygen isreleased). Due to entry of hydrogen into the oxygen vacancy, an electronserving as a carrier is generated. Further, in some cases, bonding ofpart of hydrogen to oxygen bonded to a metal element causes generationof an electron serving as a carrier. Thus, a transistor including anoxide semiconductor that contains hydrogen is likely to be normally on.

Accordingly, it is preferable that hydrogen be reduced as much aspossible as well as the oxygen vacancies in the oxide semiconductor film17. Specifically, in the oxide semiconductor film 17, the hydrogenconcentration that is measured by secondary ion mass spectrometry (SIMS)is set to 2×10²⁰ atoms/cm³ or lower, preferably 5×10¹⁹ atoms/cm³ orlower, more preferably 1×10¹⁹ atoms/cm³ or lower, more preferably 5×10¹⁸atoms/cm³ or lower, more preferably 1×10¹⁸ atoms/cm³ or lower, morepreferably 5×10¹⁷ atoms/cm³ or lower, more preferably 1×10¹⁶ atoms/cm³or lower. As a result, the transistor 10 has positive threshold voltage(normally-off characteristics).

When silicon or carbon that is one of elements belonging to Group 14 iscontained in the oxide semiconductor film 17, oxygen vacancies areincreased in the oxide semiconductor film 17, and the oxidesemiconductor film 17 becomes an n-type film. Thus, the concentration ofsilicon or carbon (the concentration is measured by SIMS) of the oxidesemiconductor film 17 is lower than or equal to 2×10¹⁸ atoms/cm³,preferably lower than or equal to 2×10¹⁷ atoms/cm³. As a result, thetransistor 10 has positive threshold voltage (normally-offcharacteristics).

Further, the concentration of alkali metal or alkaline earth metal ofthe oxide semiconductor film 17, which is measured by SIMS, is lowerthan or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal might generatecarriers when bonded to an oxide semiconductor, in which case theoff-state current of the transistor might be increased. Thus, it ispreferable to reduce the concentration of alkali metal or alkaline earthmetal of the oxide semiconductor film 17. As a result, the transistor 10has positive threshold voltage (normally-off characteristics).

Furthermore, when containing nitrogen, the oxide semiconductor film 17easily becomes an n-type film by generation of electrons serving ascarriers and an increase of carrier density. Thus, a transistorincluding an oxide semiconductor that contains nitrogen is likely to benormally on. For this reason, nitrogen in the oxide semiconductor filmis preferably reduced as much as possible; the concentration of nitrogenthat is measured by SIMS is preferably set to, for example, lower thanor equal to 5×10¹⁸ atoms/cm³.

When impurities in the oxide semiconductor film 17 are reduced, thecarrier density of the oxide semiconductor film 17 can be lowered. Theoxide semiconductor preferably has a carrier density of 1×10¹⁷/cm³ orless, more preferably 1×10¹⁵/cm³ or less, still more preferably1×10¹³/cm³ or less, yet more preferably 1×10¹¹/cm³ or less.

Note that it is preferable to use, as the oxide semiconductor film 17,an oxide semiconductor film in which the impurity concentration is lowand density of defect states is low, in which case the transistor canhave more excellent electrical characteristics. Here, the state in whichimpurity concentration is low and density of defect states is low (thenumber of oxygen vacancies is small) is referred to as “highly purifiedintrinsic” or “substantially highly purified intrinsic”. A highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor has few carrier generation sources, and thus has a lowcarrier density in some cases. Thus, a transistor including the oxidesemiconductor film in which a channel region is formed is likely to havepositive threshold voltage (normally-off characteristics). A highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor film has a low density of defect states and accordinglyhas a low trap state in some cases. Further, a highly purified intrinsicor substantially highly purified intrinsic oxide semiconductor film hasan extremely low off-state current; the off-state current can be lessthan or equal to the measurement limit of a semiconductor parameteranalyzer, i.e., less than or equal to 1×10⁻¹³ A, at a voltage (drainvoltage) between a source electrode and a drain electrode of from 1 V to10 V. Thus, the transistor whose channel region is formed in the oxidesemiconductor film has a small variation in electrical characteristicsand high reliability in some cases.

The oxide semiconductor film 17 may have a non-single-crystal structure,for example. The non-single crystal structure includes a c-axis alignedcrystalline oxide semiconductor (CAAC-OS) that is described later, apolycrystalline structure, a microcrystalline structure described later,or an amorphous structure, for example. Among the non-single crystalstructure, the amorphous structure has the highest density of defectlevels, whereas CAAC-OS has the lowest density of defect levels.

Note that the oxide semiconductor film 17 may be a mixed film includingtwo or more of the following: a region having an amorphous structure, aregion having a microcrystalline structure, a region having apolycrystalline structure, a region of CAAC-OS described later, and aregion having a single-crystal structure. The mixed film has asingle-layer structure including, for example, two or more of a regionhaving an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure in some cases.Further, the mixed film has a stacked-layer structure of two or more ofa region having an amorphous structure, a region having amicrocrystalline structure, a region having a polycrystalline structure,a CAAC-OS region, and a region having a single-crystal structure in somecases.

The pair of electrodes 19 and 20 is formed with a single-layer structureor a layered structure using any of metals such as aluminum, titanium,chromium, nickel, copper, yttrium, zirconium, molybdenum, silver,tantalum, and tungsten and an alloy containing any of these metals as amain component. For example, a single-layer structure of an aluminumfilm containing silicon, a two-layer structure in which an aluminum filmis stacked over a titanium film, a two-layer structure in which analuminum film is stacked over a tungsten film, a two-layer structure inwhich a copper film is stacked over a copper-magnesium-aluminum alloyfilm, a two-layer structure in which a copper film is stacked over atitanium film, a two-layer structure in which a copper film is stackedover a tungsten film, a three-layer structure in which a titanium filmor a titanium nitride film, an aluminum film or a copper film, and atitanium film or a titanium nitride film are stacked in this order, athree-layer structure in which a molybdenum film or a molybdenum nitridefilm, an aluminum film or a copper film, and a molybdenum film or amolybdenum nitride film are stacked in this order, and the like can begiven. Note that a transparent conductive material containing indiumoxide, tin oxide, or zinc oxide may be used.

Note that although the pair of electrodes 19 and 20 is provided betweenthe oxide semiconductor film 17 and the protective film 21 in thisembodiment, the pair of electrodes 19 and 20 may be provided between thegate insulating film 15 and the oxide semiconductor film 17.

When the gate insulating film 15 is formed of an oxide insulating filmcontaining nitrogen and having a small number of defects, the protectivefilm 21 can be formed using silicon oxide, silicon oxynitride,Ga—Zn-based metal oxide, or the like.

Further, it is possible to prevent outward diffusion of oxygen from theoxide semiconductor film 17 and entry of hydrogen, water, or the likeinto the oxide semiconductor film 17 from the outside by providing aninsulating film having a blocking effect against oxygen, hydrogen,water, and the like for the protective film 21. As for the insulatingfilm having a blocking effect against oxygen, hydrogen, water, and thelike, an aluminum oxide film, an aluminum oxynitride film, a galliumoxide film, a gallium oxynitride film, an yttrium oxide film, an yttriumoxynitride film, a hafnium oxide film, a hafnium oxynitride film, and asilicon nitride film, can be given as examples.

The thickness of the protective film 21 is preferably greater than orequal to 150 nm and less than or equal to 400 nm.

<2. Method for Manufacturing Transistor>

Next, a method for manufacturing the transistor 10 in FIGS. 1A to 1C isdescribed with reference to FIGS. 2A to 2D. A cross-section in thechannel length direction along dot-dashed line A-B in FIG. 1A and across-section in the channel width direction along dot-dashed line C-Din FIG. 1A are used in FIGS. 2A to 2D to describe the method formanufacturing the transistor 10.

As illustrated in FIG. 2A, the gate electrode 13 is formed over thesubstrate 11.

A formation method of the gate electrode 13 is described below. First, aconductive film is formed by a sputtering method, a CVD method, anevaporation method, or the like and then a mask is formed over theconductive film by a photolithography process. Next, the conductive filmis partly etched using the mask to form the gate electrode 13. Afterthat, the mask is removed.

Note that the gate electrode 13 may be formed by an electrolytic platingmethod, a printing method, an ink jet method, or the like instead of theabove formation method.

Here, a 100-nm-thick tungsten film is formed by a sputtering method.Next, a mask is formed by a photolithography process, and the tungstenfilm is subjected to dry etching with the use of the mask to form thegate electrode 13.

Then, the gate insulating film 15 is formed over the substrate 11 andthe gate electrode 13, and the oxide semiconductor film 17 is formed ina region that is over the gate insulating film 15 and overlaps with thegate electrode 13.

The gate insulating film 15 is formed by a sputtering method, a CVDmethod, an evaporation method, or the like.

In the case of forming a silicon oxide film or a silicon oxynitride filmas the gate insulating film 15, a deposition gas containing silicon andan oxidizing gas are preferably used as a source gas. Typical examplesof the deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. Examples of the oxidizing gas includeoxygen, ozone, dinitrogen monoxide, and nitrogen dioxide.

In the case where a gallium oxide film is formed as the gate insulatingfilm 15, a metal organic chemical vapor deposition (MOCVD) method can beused.

Here, a silicon oxynitride film is formed as the gate insulating film 15by a plasma CVD method.

A formation method of the oxide semiconductor film 17 is describedbelow. An oxide semiconductor film is formed over the gate insulatingfilm 15 by a sputtering method, a coating method, a pulsed laserdeposition method, a laser ablation method, or the like. Then, after amask is formed over the oxide semiconductor film by a photolithographyprocess, the oxide semiconductor film is partly etched using the mask.Accordingly, the oxide semiconductor film 17 that is over the gateinsulating film 15 and subjected to element isolation so as to partlyoverlap with the gate electrode 13 is formed as illustrated in FIG. 2B.After that, the mask is removed.

Alternatively, by using a printing method for forming the oxidesemiconductor film 17, the oxide semiconductor film 17 subjected toelement isolation can be formed directly.

As a power supply device for generating plasma in the case of formingthe oxide semiconductor film by a sputtering method, an RF power supplydevice, an AC power supply device, a DC power supply device, or the likecan be used as appropriate.

As a sputtering gas, a rare gas (typically argon), an oxygen gas, or amixed gas of a rare gas and oxygen is used as appropriate. In the caseof the mixed gas of a rare gas and oxygen, the proportion of oxygen to arare gas is preferably increased.

Further, a target may be appropriately selected in accordance with thecomposition of the oxide semiconductor film to be formed.

For example, in the case where the oxide semiconductor film is formed bya sputtering method at a substrate temperature higher than or equal to150° C. and lower than or equal to 750° C., preferably higher than orequal to 150° C. and lower than or equal to 450° C., more preferablyhigher than or equal to 200° C. and lower than or equal to 350° C., theoxide semiconductor film can be a CAAC-OS film.

For the deposition of the CAAC-OS film, the following conditions arepreferably used.

By suppressing entry of impurities into the CAAC-OS film during thedeposition, the crystal state can be prevented from being broken by theimpurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, or nitrogen) that exist in thedeposition chamber may be reduced. Furthermore, the concentration ofimpurities in a deposition gas may be reduced. Specifically, adeposition gas whose dew point is −80° C. or lower, preferably −100° C.or lower is used.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is higher than or equal to 30 vol %, preferably 100 vol%.

After the oxide semiconductor film is formed, dehydrogenation ordehydration may be performed by heat treatment. The temperature of theheat treatment is typically higher than or equal to 150° C. and lowerthan the strain point of the substrate, preferably higher than or equalto 250° C. and lower than or equal to 450° C., more preferably higherthan or equal to 300° C. and lower than or equal to 450° C.

The heat treatment is performed under an inert gas atmosphere containingnitrogen or a rare gas such as helium, neon, argon, xenon, or krypton.Further, the heat treatment may be performed under an inert gasatmosphere first, and then under an oxygen atmosphere. It is preferablethat the above inert gas atmosphere and the above oxygen atmosphere donot contain hydrogen, water, and the like. The treatment time is 3minutes to 24 hours.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With the use of an RTA apparatus, the heat treatment canbe performed at a temperature of higher than or equal to the strainpoint of the substrate if the heating time is short. Therefore, the heattreatment time can be shortened.

By forming the oxide semiconductor film while it is heated or performingheat treatment after the formation of the oxide semiconductor film, thehydrogen concentration can be 5×10¹⁹ atoms/cm³ or lower, preferably1×10¹⁹ atoms/cm³ or lower, preferably 5×10¹⁸ atoms/cm³ or lower, morepreferably 1×10¹⁸ atoms/cm³ or lower, more preferably 5×10¹⁷ atoms/cm³or lower, more preferably 1×10¹⁶ atoms/cm³ or lower.

Here, a 35-nm-thick oxide semiconductor film is formed by a sputteringmethod, a mask is formed over the oxide semiconductor film, and thenpart of the oxide semiconductor film is selectively etched. Then, afterthe mask is removed, heat treatment is performed in a mixed atmospherecontaining nitrogen and oxygen, whereby the oxide semiconductor film 17is formed.

Next, as illustrated in FIG. 2C, the pair of electrodes 19 and 20 areformed.

A method for forming the pair of electrodes 19 and 20 is describedbelow. First, a conductive film is formed by a sputtering method, a CVDmethod, an evaporation method, or the like. Then, a mask is formed overthe conductive film by a photolithography process. After that, theconductive film is etched using the mask to form the pair of electrodes19 and 20. After that, the mask is removed.

Here, a 50-nm-thick tungsten film, a 400-nm-thick aluminum film, and a100-nm-thick titanium film are sequentially stacked by a sputteringmethod. Next, a mask is formed over the titanium film by aphotolithography process and the tungsten film, the aluminum film, andthe titanium film are dry-etched with use of the mask to form the pairof electrodes 19 and 20.

Note that heat treatment may be performed after the pair of electrodes19 and 20 are formed. For example, this heat treatment can be performedin a manner similar to that of the heat treatment performed after theoxide semiconductor film 17 is formed.

After the pair of electrodes 19 and 20 are formed, cleaning treatment ispreferably performed to remove an etching residue. A short circuit ofthe pair of electrodes 19 and 20 can be suppressed by this cleaningtreatment. The cleaning treatment can be performed using an alkalinesolution such as a tetramethylammonium hydroxide (TMAH) solution; anacidic solution such as a hydrofluoric acid, an oxalic acid solution, ora phosphoric acid solution; or water.

Next, the protective film 21 is formed over the oxide semiconductor film17 and the pair of electrodes 19 and 20. The protective film 21 can beformed by a sputtering method, a CVD method, an evaporation method, orthe like.

In the case where an oxide insulating film containing nitrogen andhaving a small number of defects is formed as the protective film 21, asilicon oxynitride film can be formed by a CVD method as an example ofthe oxide insulating film. In this case, a deposition gas containingsilicon and an oxidizing gas are preferably used as a source gas.Typical examples of the deposition gas containing silicon includesilane, disilane, trisilane, and silane fluoride. Examples of theoxidizing gas include dinitrogen monoxide and nitrogen dioxide.

The oxide insulating film containing nitrogen and having a small numberof defects can be formed by a CVD method under the conditions where theratio of an oxidizing gas to a deposition gas is higher than 20 timesand lower than 100 times, preferably higher than or equal to 40 timesand lower than or equal to 80 times and the pressure in a treatmentchamber is lower than 100 Pa, preferably lower than or equal to 50 Pa.

Here, a silicon oxynitride film is formed by a plasma CVD method underthe conditions where the substrate 11 is held at a temperature of 220°C., silane at a flow rate of 50 sccm and dinitrogen monoxide at a flowrate of 2000 sccm are used as a source gas, the pressure in thetreatment chamber is 20 Pa, and a high-frequency power of 100 W at 13.56MHz (1.6×10⁻² W/cm² as the power density) is supplied to parallel-plateelectrodes.

Next, heat treatment may be performed. The temperature of the heattreatment is typically higher than or equal to 150° C. and lower thanthe strain point of the substrate, preferably higher than or equal to200° C. and lower than or equal to 450° C., further preferably higherthan or equal to 300° C. and lower than or equal to 450° C. By the heattreatment, water, hydrogen, and the like contained in the protectivefilm 21 can be released.

Here, heat treatment is performed at 350° C. in a mixed atmospherecontaining nitrogen and oxygen for one hour.

Through the above steps, a transistor in which a change in thresholdvoltage is reduced can be manufactured. Further, a transistor in which achange in electrical characteristics is reduced can be manufactured.

Modification Example 1

Modification examples of the transistor 10 described in Embodiment 1 aredescribed with reference to FIGS. 3A and 3B. In each of the transistorsdescribed in this modification example, a gate insulating film or aprotective film has a stacked-layer structure.

In a transistor using an oxide semiconductor, oxygen vacancies in anoxide semiconductor film cause defects of electrical characteristics ofthe transistor. For example, the threshold voltage of a transistorincluding an oxide semiconductor film that contains oxygen vacancies inthe film easily shifts in the negative direction, and such a transistortends to have normally-on characteristics. This is because charge isgenerated owing to the oxygen vacancies in the oxide semiconductor film,resulting in reduction of the resistance of the oxide semiconductorfilm.

Further, when an oxide semiconductor film includes oxygen vacancies,there is a problem in that the amount of change in electricalcharacteristics, typically change of the threshold voltage of thetransistor is increased due to change over time or a bias-temperaturestress test (hereinafter also referred to as a BT stress test).

Thus, by forming an oxide insulating film containing oxygen at a higherproportion than oxygen in the stoichiometric composition as a part ofthe protective film, a transistor in which a shift of the thresholdvoltage in the negative direction is suppressed and that has excellentelectrical characteristics can be manufactured. In addition, a highlyreliable transistor in which a variation in electrical characteristicswith time or a variation in electrical characteristics due to a gate BTphotostress test is small can be manufactured.

In a transistor 10 a illustrated in FIG. 3A, the protective film 21 hasa multi-layer structure. Specifically, the protective film 21 includesan oxide insulating film 23, an oxide insulating film 25 containingoxygen at a higher proportion than oxygen in the stoichiometriccomposition, and a nitride insulating film 27. The oxide insulating film23 in contact with the oxide semiconductor film 17 is an oxideinsulating film containing nitrogen and having a small number of defectsthat can be used as at least one of the gate insulating film 15 and theprotective film 21 of the transistor 10.

The oxide insulating film 25 is formed using an oxide insulating filmthat contains oxygen at a higher proportion than oxygen in thestoichiometric composition. Part of oxygen is released by heating fromthe oxide insulating film containing oxygen at a higher proportion thanoxygen in the stoichiometric composition. The oxide insulating filmcontaining oxygen at a higher proportion than oxygen in thestoichiometric composition is an oxide insulating film of which theamount of released oxygen converted into oxygen atoms is greater than orequal to 1.0×10¹⁸ atoms/cm³, preferably greater than or equal to3.0×10²⁰ atoms/cm³ in TDS analysis. Note that the substrate temperaturein the TDS analysis is preferably higher than or equal to 100° C. andlower than or equal to 700° C., or higher than or equal to 100° C. andlower than or equal to 500° C.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 30 nm and less than or equal to 500nm, or greater than or equal to 50 nm and less than or equal to 400 nmcan be used for the oxide insulating film 25.

As the oxide insulating film 25, a silicon oxide film or a siliconoxynitride film is formed under the following conditions: the substrateplaced in a treatment chamber of the plasma CVD apparatus that isvacuum-evacuated is held at a temperature higher than or equal to 180°C. and lower than or equal to 280° C., preferably higher than or equalto 200° C. and lower than or equal to 240° C., the pressure is greaterthan or equal to 100 Pa and less than or equal to 250 Pa, preferablygreater than or equal to 100 Pa and less than or equal to 200 Pa withintroduction of a source gas into the treatment chamber, and ahigh-frequency power of greater than or equal to 0.17 W/cm² and lessthan or equal to 0.5 W/cm², preferably greater than or equal to 0.25W/cm² and less than or equal to 0.35 W/cm² is supplied to an electrodeprovided in the treatment chamber.

As a source gas of the oxide insulating film 25, a deposition gascontaining silicon and an oxidizing gas is preferably used. Typicalexamples of the deposition gas containing silicon include silane,disilane, trisilane, and silane fluoride. As the oxidizing gas, oxygen,ozone, dinitrogen monoxide, and nitrogen dioxide can be given asexamples.

As the film formation conditions of the oxide insulating film 25, thehigh-frequency power having the above power density is supplied to atreatment chamber having the above pressure, whereby the degradationefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; thus, the oxygencontent in the oxide insulating film 25 becomes higher than that in thestoichiometric composition. On the other hand, in the film formed at asubstrate temperature within the above temperature range, the bondbetween silicon and oxygen is weak, and accordingly, part of oxygen inthe film is released by heat treatment in a later step. Thus, it ispossible to form an oxide insulating film which contains oxygen at ahigher proportion than oxygen in the stoichiometric composition and fromwhich part of oxygen is released by heating. Further, the oxideinsulating film 23 is provided over the oxide semiconductor film 17.Accordingly, in the step of forming the oxide insulating film 25, theoxide insulating film 23 serves as a protective film of the oxidesemiconductor film 17. Consequently, the oxide insulating film 25 can beformed using the high-frequency power having a high power density whiledamage to the oxide semiconductor film 17 is reduced. By the later heattreatment step, part of oxygen contained in the oxide insulating film 25can be moved to the oxide semiconductor film 17, so that oxygenvacancies contained in the oxide semiconductor film 17 can be furtherreduced.

As the nitride insulating film 27, a film having an effect of blockingat least hydrogen and oxygen is used. Preferably, the nitride insulatingfilm 27 has an effect of blocking oxygen, hydrogen, water, an alkalimetal, an alkaline earth metal, or the like. It is possible to preventoutward diffusion of oxygen from the oxide semiconductor film 17 andentry of hydrogen, water, or the like into the oxide semiconductor film17 from the outside by providing the nitride insulating film 27.

The nitride insulating film 27 is formed using a silicon nitride film, asilicon nitride oxide film, an aluminum nitride film, an aluminumnitride oxide film, or the like having a thickness greater than or equalto 50 nm and less than or equal to 300 nm, preferably greater than orequal to 100 nm and less than or equal to 200 nm.

Note that instead of the nitride insulating film 27, an oxide insulatingfilm having a blocking effect against oxygen, hydrogen, water, and thelike may be provided. As the oxide insulating film having a blockingeffect against oxygen, hydrogen, water, and the like, an aluminum oxidefilm, an aluminum oxynitride film, a gallium oxide film, a galliumoxynitride film, an yttrium oxide film, an yttrium oxynitride film, ahafnium oxide film, and a hafnium oxynitride film can be given.

The nitride insulating film 27 can be formed by a sputtering method, aCVD method, or the like.

In the case where a silicon nitride film is formed by the plasma CVDmethod as the nitride insulating film 27, a deposition gas containingsilicon, nitrogen, and ammonia is used as the source gas. As the sourcegas, ammonia whose amount is smaller than the amount of nitrogen isused, whereby ammonia is dissociated in the plasma and activated speciesare generated. The activated species break a bond between silicon andhydrogen that are contained in a deposition gas containing silicon and atriple bond between nitrogen molecules. As a result, a dense siliconnitride film having few defects, in which bonds between silicon andnitrogen are promoted and bonds between silicon and hydrogen is few, canbe formed. On the other hand, when the amount of ammonia is larger thanthe amount of nitrogen in a source gas, dissociation of a deposition gascontaining silicon and decomposition of nitrogen are not promoted, sothat a sparse silicon nitride film in which bonds between silicon andhydrogen remain and defects are increased is formed. Therefore, in asource gas, the flow ratio of the nitrogen to the ammonia is set to bepreferably greater than or equal to 5 and less than or equal to 50, morepreferably greater than or equal to 10 and less than or equal to 50.

In a transistor 10 b illustrated in FIG. 3B, the gate insulating film 15has a stacked structure of a nitride insulating film 29 and an oxideinsulating film 31 containing nitrogen, and the oxide insulating film 31in contact with the oxide semiconductor film 17 is an oxide insulatingfilm containing nitrogen and having a small number of defects.

As the nitride insulating film 29, a film having an effect of blockingwater, hydrogen, or the like is preferably used. Alternatively, as thenitride insulating film 29, a film with a small number of defects ispreferably used. Typical examples of the nitride insulating film 29include films of silicon nitride, silicon nitride oxide, aluminumnitride, aluminum nitride oxide, and the like.

The use of a silicon nitride film as the nitride insulating film 29 hasthe following effect. In addition, a silicon nitride film has a higherdielectric constant than a silicon oxide film and needs a largerthickness for capacitance equivalent to that of the silicon oxide. Thus,the physical thickness of the gate insulating film 15 can be increased.This makes it possible to reduce a decrease in withstand voltage of thetransistor 10 b and furthermore increase the withstand voltage, therebyreducing electrostatic discharge damage to a semiconductor device.

In the transistor including the oxide semiconductor film, when trapstates (also referred to as interface states) are included in the gateinsulating film 15, the trap states can cause a change in electricalcharacteristics, typically, a change in the threshold voltage, of thetransistor. As a result, there is a problem in that electricalcharacteristics vary among transistors. Therefore, with the use of asilicon nitride film having few defects as the nitride insulating film29, the shift of the threshold voltage and the variation in theelectrical characteristics among transistors can be reduced.

The nitride insulating film 29 may have a stacked-layer structure. Forexample, the nitride insulating film 29 has a stacked structure in whicha first silicon nitride film is formed using a silicon nitride filmhaving fewer defects, and a second silicon nitride film using a siliconnitride film that releases a small number of hydrogen molecules andammonia molecules is formed over the first silicon nitride film, wherebythe gate insulating film 15 can be formed using a gate insulating filmthat has fewer defects and releases a small number of hydrogen moleculesand ammonia molecules. As a result, movement of hydrogen and nitrogencontained in the gate insulating film 15 to the oxide semiconductor film17 can be suppressed.

The nitride insulating film 29 is preferably formed by stacking siliconnitride films by a two-step formation method. First, a first siliconnitride film with a small number of defects is formed by a plasma CVDmethod in which a mixed gas of silane, nitrogen, and ammonia is used asa source gas. Then, by using a source gas at a flow ratio that issimilar to that of a source gas used for the nitride insulating film 27,a silicon nitride film that releases a small number of hydrogenmolecules and ammonia molecules can be formed as the second siliconnitride film.

Modification Example 2

Modification examples of the transistor 10 described in Embodiment 1 aredescribed with reference to FIGS. 4A and 4B. The transistor 10 describedin Embodiment 1 is a channel-etched transistor; in contrast, atransistor 10 c described in this modification example is achannel-protective transistor.

The transistor 10 c illustrated in FIG. 4A includes the gate electrode13 over the substrate 11; the gate insulating film 15 over the substrate11 and the gate electrode 13; the oxide semiconductor film 17overlapping with the gate electrode 13 with the gate insulating film 15therebetween; an insulating film 33 over the gate insulating film 15 andthe oxide semiconductor film 17; and the pair of electrodes 19 and 20 incontact with the oxide semiconductor film 17 in openings of theinsulating film 33.

A transistor 10 d illustrated in FIG. 4B includes an insulating film 35over the oxide semiconductor film 17 and the pair of electrodes 19 and20 in contact with the oxide semiconductor film 17.

In the transistor 10 c or 10 d, part of the oxide semiconductor film 17,typically, a back channel region is covered with the insulating film 33or 35; accordingly, the back channel region of the oxide semiconductorfilm 17 is not damaged by etching for forming the pair of electrodes 19and 20. In addition, when the insulating film 33 or 35 is an oxideinsulating film containing nitrogen and having a small number ofdefects, a change in electrical characteristics is suppressed, wherebythe transistor can have improved reliability.

Modification Example 3

Modification examples of the transistor 10 described in Embodiment 1 aredescribed with reference to FIGS. 5A to 5C. The transistor 10 describedin Embodiment 1 includes one gate electrode; in contrast, a transistor10 e described in this modification example includes two gate electrodeswith an oxide semiconductor film interposed between the gate electrodes.

A top view and cross-sectional views of the transistor 10 e included ina semiconductor device are illustrated in FIGS. 5A to 5C. FIG. 5A is atop view of the transistor 10 e, FIG. 5B is a cross-sectional view takenalong dashed-dotted line A-B in FIG. 5A, and FIG. 5C is across-sectional view taken along dashed-dotted line C-D in FIG. 5A. Notethat in FIG. 5A, the substrate 11, the gate insulating film 15, theprotective film 21, and the like are omitted for simplicity.

The transistor 10 e illustrated in FIGS. 5B and 5C is a channel-etchedtransistor including the gate electrode 13 over the substrate 11; thegate insulating film 15 formed over the substrate 11 and the gateelectrode 13; the oxide semiconductor film 17 overlapping with the gateelectrode 13 with the gate insulating film 15 provided therebetween; andthe pair of electrodes 19 and 20 in contact with the oxide semiconductorfilm 17. The transistor 10 e further includes the protective film 21including the oxide insulating film 23, the oxide insulating film 25,and the nitride insulating film 27 over the gate insulating film 15, theoxide semiconductor film 17, and the pair of electrodes 19 and 20; and agate electrode 37 over the protective film 21. The gate electrode 37 isconnected to the gate electrode 13 through openings 42 and 43 providedin the gate insulating film 15 and the protective film 21. Here, thegate insulating film 15 is a stack of the nitride insulating film 29 andoxide insulating film 31. The protective film 21 is a stack of the oxideinsulating film 23, the oxide insulating film 25, and the nitrideinsulating film 27.

A plurality of openings are provided in the gate insulating film 15 andthe protective film 21. Typically, the openings 42 and 43 are providedwith the oxide semiconductor film 17 provided therebetween in thechannel width direction as illustrated in FIG. 5C. In other words, theopenings 42 and 43 are provided on outer sides of the side surfaces ofthe oxide semiconductor film 17. In addition, in the openings 42 and 43,the gate electrode 13 is connected to the gate electrode 37. This meansthat the gate electrode 13 and the gate electrode 37 surround the oxidesemiconductor film 17 in the channel width direction with the gateinsulating film 15 and the protective film 21 provided between the oxidesemiconductor film 17 and each of the gate electrode 13 and the gateelectrode 37. Furthermore, in the channel width direction, the gateelectrode 37 in the openings 42 and 43 and each of the side surfaces ofthe oxide semiconductor film 17 are provided so that the protective film21 is positioned therebetween.

As illustrated in FIG. 5C, a side surface of the oxide semiconductorfilm 17 faces the gate electrode 37 in the channel width direction, andthe oxide semiconductor film 17 is surrounded by the gate electrode 13and the gate electrode 37 with the gate insulating film 15 interposedbetween the oxide semiconductor film 17 and the gate electrode 13 andthe protective film 21 interposed between the oxide semiconductor film17 and the gate electrode 37 in the channel width direction. Thus, inthe oxide semiconductor film 17, carriers flow not only at the interfacebetween the gate insulating film 15 and the oxide semiconductor film 17and the interface between the protective film 21 and the oxidesemiconductor film 17, but also in the oxide semiconductor film 17,whereby the amount of transfer of carriers is increased in thetransistor 10 e. As a result, the on-state current and field-effectmobility of the transistor 10 are increased. The electric field of thegate electrode 37 affects the side surface or an end portion includingthe side surface and its vicinity of the oxide semiconductor film 17;thus, generation of a parasitic channel at the side surface or the endportion of the oxide semiconductor film 17 can be suppressed.

Modification Example 4

Modification examples of the transistor 10 described in Embodiment 1 aredescribed with reference to FIGS. 6A to 6D and FIGS. 7A to 7C. Thetransistor 10 described in Embodiment 1 includes the single-layer oxidesemiconductor film; in contrast, transistors 10 f and 10 g described inthis modification example each includes a multi-layer film.

FIGS. 6A to 6C are a top view and cross-sectional views of thetransistor 10 f included in a semiconductor device. FIG. 6A is a topview of the transistor 10 f, FIG. 6B is a cross-sectional view takenalong dashed-dotted line A-B in FIG. 6A, and FIG. 6C is across-sectional view taken along dashed-dotted line C-D in FIG. 6A. Notethat in FIG. 6A, the substrate 11, the gate insulating film 15, theprotective film 21, and the like are omitted for simplicity.

The transistor 10 f illustrated in FIG. 6A includes a multilayer film 45overlapping with the gate electrode 13 with the gate insulating film 15provided therebetween, and the pair of electrodes 19 and 20 in contactwith the multilayer film 45. The protective film 21 is stacked over thegate insulating film 15, the multilayer film 45, and the pair ofelectrodes 19 and 20.

In the transistor 10 f described in this embodiment, the multilayer film45 includes the oxide semiconductor film 17 and an oxide semiconductorfilm 46. That is, the multilayer film 45 has a two-layer structure.Furthermore, part of the oxide semiconductor film 17 serves as a channelregion. In addition, a protective film is formed in contact with themultilayer film 45.

The oxide semiconductor film 46 contains one or more elements that formthe oxide semiconductor film 17. Thus, interface scattering is unlikelyto occur at the interface between the oxide semiconductor film 17 andthe oxide semiconductor film 46. Thus, the transistor can have highfield-effect mobility because the movement of carriers is not hinderedat the interfaces.

The oxide semiconductor film 46 is formed using a metal oxide filmcontaining at least In or Zn. Typical examples of the metal oxide filminclude an In—Ga oxide film, an In—Zn oxide film, and an In-M-Zn oxidefilm (M represents Al, Ga, Y, Zr, La, Ce, or Nd). The conduction bandminimum of the oxide semiconductor film 46 is closer to a vacuum levelthan that of the oxide semiconductor film 17 is; as a typical example,the energy difference between the conduction band minimum of the oxidesemiconductor film 46 and the conduction band minimum of the oxidesemiconductor film 17 is any one of 0.05 eV or more, 0.07 eV or more,0.1 eV or more, or 0.15 eV or more, and any one of 2 eV or less, 1 eV orless, 0.5 eV or less, or 0.4 eV or less. That is, the difference betweenthe electron affinity of the oxide semiconductor film 46 and theelectron affinity of the oxide semiconductor film 17 is any one of 0.05eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more, and anyone of 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV or less.

The oxide semiconductor film 46 preferably contains In because carriermobility (electron mobility) can be increased.

When the oxide semiconductor film 46 contains a larger amount of Al, Ga,Y, Zr, La, Ce, or Nd than the amount of In in an atomic ratio, any ofthe following effects may be obtained: (1) the energy gap of the oxidesemiconductor film 46 is widened; (2) the electron affinity of the oxidesemiconductor film 46 decreases; (3) impurity diffusion from the outsideis suppressed; (4) an insulating property of the oxide semiconductorfilm 46 increases as compared to that of the oxide semiconductor film17; and (5) an oxygen vacancy is less likely to be generated because Al,Ga, Y, Zr, La, Ce, or Nd is a metal element that is strongly bonded tooxygen.

In the case where the oxide semiconductor film 46 is an In-M-Zn oxidefilm, the proportion of In and the proportion of M, not taking Zn and Ointo consideration, are less than 50 atomic % and greater than or equalto 50 atomic %, respectively, and preferably less than 25 atomic % andgreater than or equal to 75 atomic %, respectively.

Furthermore, in the case where each of the oxide semiconductor films 17and 46 contains an In-M-Zn oxide (M represents Al, Ga, Y, Zr, La, Ce, orNd), the proportion of M atoms (M represents Al, Ga, Y, Zr, La, Ce, orNd) in the oxide semiconductor film 46 is higher than that in the oxidesemiconductor film 17. As a typical example, the proportion of M in theoxide semiconductor film 17 is 1.5 or more times, preferably twice ormore, further preferably three or more times as high as that in theoxide semiconductor film 17.

Furthermore, in the case where each of the oxide semiconductor films 17and 46 contains an In-M-Zn oxide (M represents Al, Ga, Y, Zr, La, Ce, orNd), when In:M:Zn=x₁:y₁:z₁ [atomic ratio] is satisfied in the oxidesemiconductor film 46 and In:M:Zn=x₂:y₂:z₂ [atomic ratio] is satisfiedin the oxide semiconductor film 17, y₁/x₁ is higher than y₂/x₂, andpreferably, y₁/x₁ be 1.5 or more times as high as y₂/x₂. Alternatively,y₁/x₁ is preferably twice or more as high as y₂/x₂. Furtheralternatively, y₁/x₁ is preferably three or more times as high as y₂/x₂.In this case, it is preferable that in the oxide semiconductor film, y₂be higher than or equal to x₂ because a transistor including the oxidesemiconductor film can have stable electrical characteristics.

In the case where the oxide semiconductor film 17 is an In-M-Zn oxidefilm (M is Al, Ga, Y, Zr, La, Ce, or Nd) and a target having the atomicratio of metal elements of In:M:Zn=x₁:y₁:z₁ is used for forming theoxide semiconductor film 17, x₁/y₁ is preferably greater than or equalto ⅓ and less than or equal to 6, further preferably greater than orequal to 1 and less than or equal to 6, and z₁/y₁ is preferably greaterthan or equal to ⅓ and less than or equal to 6, further preferablygreater than or equal to 1 and less than or equal to 6. Note that whenz₁/y₁ is greater than or equal to 1 and less than or equal to 6, aCAAC-OS film to be described later as the oxide semiconductor film 17 iseasily formed. Typical examples of the atomic ratio of the metalelements of the target are In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, andIn:M:Zn=3:1:2.

In the case where the oxide semiconductor film 46 is an In-M-Zn oxidefilm (M is Al, Ga, Y, Zr, La, Ce, or Nd) and a target having the atomicratio of metal elements of In:M:Zn=x₂:y₂:z₂ is used for forming theoxide semiconductor film 46, x₂/y₂ is preferably less than x₁/y₁, andz₂/y₂ is preferably greater than or equal to ⅓ and less than or equal to6, further preferably greater than or equal to 1 and less than or equalto 6. Note that when z₂/y₂ is greater than or equal to 1 and less thanor equal to 6, a CAAC-OS film to be described later as the oxidesemiconductor film 46 is easily formed. Typical examples of the atomicratio of the metal elements of the target are In:M:Zn=1:3:2,In:M:Zn=1:3:4, In:M:Zn=1:3:6, In:M:Zn=1:3:8, and the like.

Note that the proportion of each metal element in the atomic ratio ofeach of the oxide semiconductor films 17 and 46 varies within a range of±40% of that in the above atomic ratio as an error.

The thickness of the oxide semiconductor film 46 is greater than orequal to 3 nm and less than or equal to 100 nm, preferably greater thanor equal to 3 nm and less than or equal to 50 nm.

The oxide semiconductor film 46 may have a non-single-crystal structure,for example, like the oxide semiconductor film 17. The non-singlecrystal structure includes a c-axis aligned crystalline oxidesemiconductor (CAAC-OS) that is described later, a polycrystallinestructure, a microcrystalline structure described later, or an amorphousstructure, for example.

The oxide semiconductor film 46 may have an amorphous structure, forexample. An amorphous oxide semiconductor film has, for example,disordered atomic arrangement and no crystalline component.Alternatively, an amorphous oxide semiconductor film has, for example,an absolutely amorphous structure and no crystal part.

Note that the oxide semiconductor films 17 and 46 may each be a mixedfilm including two or more of a region having an amorphous structure, aregion having a microcrystalline structure, a region having apolycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure. The mixed film has a single-layer structureincluding, for example, two or more of a region having an amorphousstructure, a region having a microcrystalline structure, a region havinga polycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure in some cases. Furthermore, in some cases, themixed film has a stacked-layer structure of two or more of a regionhaving an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure.

In this case, the oxide semiconductor film 46 is provided between theoxide semiconductor film 17 and the oxide insulating film 23. Thus, ifcarrier traps are formed between the oxide semiconductor film 46 and theprotective film 21 by impurities and defects, electrons flowing in theoxide semiconductor film 17 are less likely to be trapped by the carriertraps because there is a distance between the region where the carriertraps are formed and the oxide semiconductor film 17. Accordingly, theamount of on-state current of the transistor can be increased, and thefield-effect mobility can be increased. When the electrons are trappedby the carrier traps, the electrons become negative fixed charges. As aresult, the threshold voltage of the transistor varies. However, by thedistance between the region where the carrier traps are formed and theoxide semiconductor film 17, trap of the electrons by the carrier trapscan be reduced, and accordingly fluctuations of the threshold voltagecan be reduced.

The oxide semiconductor film 46 can block impurities from the outside,and accordingly, the amount of impurities that are transferred from theoutside to the oxide semiconductor film 17 can be reduced. Furthermore,an oxygen vacancy is less likely to be formed in the oxide semiconductorfilm 46. Consequently, the impurity concentration and oxygen vacanciesin the oxide semiconductor film 17 can be reduced.

Note that the oxide semiconductor films 17 and 46 are not formed bysimply stacking each film, but are formed to form a continuous junction(here, in particular, a structure in which the energy of the conductionband minimum is changed continuously between each film). In other words,a stacked-layer structure in which there exists no impurity that forms adefect level such as a trap center or a recombination center at eachinterface is provided. If an impurity exists between the oxidesemiconductor films 17 and 46 that are stacked, a continuity of theenergy band is damaged, and the carrier is trapped or recombined at theinterface and then disappears.

To form such a continuous energy band, it is necessary to form filmscontinuously without being exposed to the air, with use of amulti-chamber deposition apparatus (sputtering apparatus) including aload lock chamber. Each chamber in the sputtering apparatus ispreferably evacuated to be a high vacuum state (to the degree of about5×10⁻⁷ Pa to 1×10⁻⁴ Pa) with an adsorption vacuum evacuation pump suchas a cryopump in order to remove water or the like, which serves as animpurity against the oxide semiconductor film, as much as possible.Alternatively, a turbo molecular pump and a cold trap are preferablycombined so as to prevent a backflow of a gas, especially a gascontaining carbon or hydrogen from an exhaust system to the inside ofthe chamber.

Note that a multilayer film 45 that is in a transistor 10 g illustratedin FIG. 6D and has three oxide semiconductor films may be includedinstead of the multilayer film 45 having two oxide semiconductor films.

An oxide semiconductor film 47, the oxide semiconductor film 17, and theoxide semiconductor film 46 are stacked in this order in the multilayerfilm 45. That is, the multilayer film 45 has a three-layer structure.Furthermore, the oxide semiconductor film 17 serves as a channel region.

The gate insulating film 15 is in contact with the oxide semiconductorfilm 47. In other words, the oxide semiconductor film 47 is providedbetween the gate insulating film 15 and the oxide semiconductor film 17.

Furthermore, the oxide semiconductor film 46 is in contact with theprotective film 21. That is, the oxide semiconductor film 46 is providedbetween the oxide semiconductor film 17 and the protective film 21.

The oxide semiconductor film 47 can be formed using a material and aformation method similar to those of the oxide semiconductor film 46.

It is preferable that the thickness of the oxide semiconductor film 47be smaller than those of the oxide semiconductor film 17. When thethickness of the oxide semiconductor film 47 is greater than or equal to1 nm and less than or equal to 5 nm, preferably greater than or equal to1 nm and less than or equal to 3 nm, the amount of change in thethreshold voltage of the transistor can be reduced.

In the transistors described in this embodiment, the oxide semiconductorfilm 46 is provided between the oxide semiconductor film 17 and theprotective film 21. Thus, if carrier traps are formed between the oxidesemiconductor film 46 and the oxide insulating film 23 by impurities anddefects, electrons flowing in the oxide semiconductor film 17 are lesslikely to be trapped by the carrier traps because there is a distancebetween the region where the carrier traps are formed and the oxidesemiconductor film 17. Accordingly, the amount of on-state current ofthe transistor can be increased, and the field-effect mobility can beincreased. When the electrons are trapped by the carrier traps, theelectrons behave as negative fixed charges. As a result, the thresholdvoltage of the transistor varies. However, by the distance between theregion where the carrier traps are formed and the oxide semiconductorfilm 17, trap of electrons by the carrier traps can be reduced, andaccordingly, fluctuations of the threshold voltage can be reduced.

The oxide semiconductor film 46 can block entry of impurities from theoutside, and accordingly, the amount of impurities transferred to theoxide semiconductor film 17 from the outside can be reduced.Furthermore, an oxygen vacancy is less likely to be formed in the oxidesemiconductor film 46. Consequently, the impurity concentration and thenumber of oxygen vacancies in the oxide semiconductor film 17 can bereduced.

The oxide semiconductor film 47 is provided between the gate insulatingfilm 15 and the oxide semiconductor film 17, and the oxide semiconductorfilm 46 is provided between the oxide semiconductor film 17 and theprotective film 21. Thus, it is possible to reduce the concentration ofsilicon or carbon in the vicinity of the interface between the oxidesemiconductor film 47 and the oxide semiconductor film 17, in the oxidesemiconductor film 17, or in the vicinity of the interface between theoxide semiconductor film 46 and the oxide semiconductor film 17.

The transistor 10 g having such a structure includes very few defects inthe multilayer film 45 including the oxide semiconductor film 17; thus,the electrical characteristics, typified by the on-state current and thefield-effect mobility, of these transistors can be improved. Further, ina gate BT stress test and a gate BT photostress test that are examplesof a stress test, the amount of change in threshold voltage is small,and thus, reliability is high.

<Band Structure of Transistor>

Next, band structures of the multilayer film 45 included in thetransistor 10 f illustrated in FIG. 6B and the multilayer film 45included in the transistor 10 g illustrated in FIG. 6D are describedwith reference to FIGS. 7A to 7C.

Here, for example, an In—Ga—Zn oxide having an energy gap of 3.15 eV isused for the oxide semiconductor film 17, and an In—Ga—Zn oxide havingan energy gap of 3.5 eV is used for the oxide semiconductor film 46. Theenergy gaps can be measured using a spectroscopic ellipsometer (UT-300manufactured by HORIBA JOBIN YVON SAS.).

The energy difference between the vacuum level and the valence bandmaximum (also called ionization potential) of the oxide semiconductorfilm 17 and the energy difference between the vacuum level and thevalence band maximum of the oxide semiconductor film 46 were 8 eV and8.2 eV, respectively. Note that the energy difference between the vacuumlevel and the valence band maximum can be measured using an ultravioletphotoelectron spectroscopy (UPS) device (VersaProbe manufactured byULVAC-PHI, Inc.).

Thus, the energy difference between the vacuum level and the conductionband minimum (also called electron affinity) of the oxide semiconductorfilm 17 and the energy difference between the vacuum level and theconduction band minimum of the oxide semiconductor film 46 are 4.85 eVand 4.7 eV, respectively.

FIG. 7A schematically illustrates a part of the band structure of themultilayer film 45 included in the transistor 10 f. Here, the case wheresilicon oxide films are used for the gate insulating film 15 and theprotective film 21 and the silicon oxide films are provided in contactwith the multilayer film 45 is described. In FIG. 7A, EcI1 denotes theenergy of the conduction band minimum of the silicon oxide film;

EcS1 denotes the energy of the conduction band minimum of the oxidesemiconductor film 17; EcS2 denotes the energy of the conduction bandminimum of the oxide semiconductor film 46; and EcI2 denotes the energyof the conduction band minimum of the silicon oxide film. Furthermore,EcI1 and EcI2 correspond to the gate insulating film 15 and theprotective film 21 in FIG. 6B, respectively.

As illustrated in FIG. 7A, there is no energy barrier between the oxidesemiconductor films 17 and 46, and the energy of the conduction bandminimum gradually changes therebetween. In other words, the energy ofthe conduction band minimum is continuously changed. This is because themultilayer film 45 contains an element contained in the oxidesemiconductor film 17 and oxygen is transferred between the oxidesemiconductor films 17 and 46, so that a mixed layer is formed.

As shown in FIG. 7A, the oxide semiconductor film 17 in the multilayerfilm 45 serves as a well and a channel region of the transistorincluding the multilayer film 45 is formed in the oxide semiconductorfilm 17. Note that since the energy of the conduction band minimum ofthe multilayer film 45 is continuously changed, it can be said that theoxide semiconductor films 17 and 46 are continuous.

Although trap levels due to impurities or defects might be generated inthe vicinity of the interface between the oxide semiconductor film 46and the protective film 21 as shown in FIG. 7A, the oxide semiconductorfilm 17 can be distanced from the region where the trap levels aregenerated owing to the existence of the oxide semiconductor film 46.However, when the energy difference between EcS1 and EcS2 is small, anelectron in the oxide semiconductor film 17 might reach the trap levelacross the energy difference. When the electron is trapped by the traplevel, a negative fixed charge is generated at the interface with theoxide insulating film, whereby the threshold voltage of the transistorshifts in the positive direction. Thus, it is preferable that the energydifference between EcS1 and EcS2 be 0.1 eV or more, further preferably0.15 eV or more, because a change in the threshold voltage of thetransistor is reduced and stable electrical characteristics areobtained.

FIG. 7B schematically illustrates a part of the band structure of themultilayer film 45 of the transistor 10 f, which is a variation of theband structure shown in FIG. 7A. Here, a structure where silicon oxidefilms are used for the gate insulating film 15 and the protective film21 and the silicon oxide films are in contact with the multilayer film45 is described. In FIG. 7B, EcI1 denotes the energy of the conductionband minimum of the silicon oxide film; EcS1 denotes the energy of theconduction band minimum of the oxide semiconductor film 17; and EcI2denotes the energy of the conduction band minimum of the silicon oxidefilm. Further, EcI1 and EcI2 correspond to the gate insulating film 15and the protective film 21 in FIG. 6B, respectively.

In the transistor illustrated in FIG. 6B, an upper portion of themultilayer film 45, that is, the oxide semiconductor film 46 might beetched in formation of the pair of electrodes 19 and 20. Furthermore, amixed layer of the oxide semiconductor films 17 and 46 is likely to beformed on the top surface of the oxide semiconductor film 17 information of the oxide semiconductor film 46.

For example, Ga content in the oxide semiconductor film 46 is higherthan that in the oxide semiconductor film 17 in the case where the oxidesemiconductor film 17 is an oxide semiconductor film formed with use of,as a sputtering target, In—Ga—Zn oxide whose atomic ratio of In to Gaand Zn is 1:1:1 or In—Ga—Zn oxide whose atomic ratio of In to Ga and Znis 3:1:2, and the oxide semiconductor film 46 is an oxide film formedwith use of, as a sputtering target, In—Ga—Zn oxide whose atomic ratioof In to Ga and Zn is 1:3:2, In—Ga—Zn oxide whose atomic ratio of In toGa and Zn is 1:3:4, or In—Ga—Zn oxide whose atomic ratio of In to Ga andZn is 1:3:6. Thus, a GaO_(x) layer or a mixed layer whose Ga content ishigher than that in the oxide semiconductor film 17 can be formed on thetop surface of the oxide semiconductor film 17.

For that reason, even in the case where the oxide semiconductor film 46is etched, the energy of the conduction band minimum EcS1 on the EcI2side is increased and the band structure shown in FIG. 7B can beobtained in some cases.

As in the band structure shown in FIG. 7B, in observation of a crosssection of a channel region, only the oxide semiconductor film 17 in themultilayer film 45 is apparently observed in some cases. However, amixed layer that contains Ga more than the oxide semiconductor film 17is formed over the oxide semiconductor film 17 in fact, and thus themixed layer can be regarded as a 1.5-th layer. Note that the mixed layercan be confirmed by analyzing a composition in the upper portion of theoxide semiconductor film 17, when the elements contained in themultilayer film 45 are measured by an EDX analysis, for example. Themixed layer can be confirmed, for example, in such a manner that the Gacontent in the composition in the upper portion of the oxidesemiconductor film 17 is larger than the Ga content in the oxidesemiconductor film 17.

FIG. 7C schematically illustrates a part of the band structure of themultilayer film 45 of the transistor 10 g. Here, the case where siliconoxide films are used for the gate insulating film 15 and the protectivefilm 21 and the silicon oxide films are in contact with the multilayerfilm 45 is described. In FIG. 7C, EcI1 denotes the energy of theconduction band minimum of the silicon oxide film; EcS1 denotes theenergy of the conduction band minimum of the oxide semiconductor film17; EcS2 denotes the energy of the conduction band minimum of the oxidesemiconductor film 46; EcS3 denotes the energy of the conduction bandminimum of the oxide semiconductor film 47; and EcI2 denotes the energyof the conduction band minimum of the silicon oxide film. Furthermore,EcI1 and EcI2 correspond to the gate insulating film 15 and theprotective film 21 in FIG. 6D, respectively.

As illustrated in FIG. 7C, there is no energy barrier between the oxidesemiconductor films 47, 17, and 46, and the conduction band minimumsthereof smoothly vary. In other words, the conduction band minimums arecontinuous. This is because the multilayer film 45 contains an elementcontained in the oxide semiconductor film 17 and oxygen is transferredbetween the oxide semiconductor films 17 and 47 and between the oxidesemiconductor films 17 and 46, so that a mixed layer is formed.

As shown in FIG. 7C, the oxide semiconductor film 17 in the multilayerfilm 45 serves as a well and a channel region of the transistorincluding the multilayer film 45 is formed in the oxide semiconductorfilm 17. Note that since the energy of the conduction band minimum ofthe multilayer film 45 is continuously changed, it can be said that theoxide semiconductor films 47, 17, and 46 are continuous.

Although trap levels due to impurities or defects might be generated inthe vicinity of the interface between the oxide semiconductor film 17and the protective film 21 and in the vicinity of the interface betweenthe oxide semiconductor film 17 and the gate insulating film 15, asillustrated in FIG. 7C, the oxide semiconductor film 17 can be distancedfrom the region where the trap levels are generated owing to theexistence of the oxide semiconductor films 46 and 47. However, when theenergy difference between EcS1 and EcS2 and the energy differencebetween EcS1 and EcS3 are small, electrons in the oxide semiconductorfilm 17 might reach the trap level across the energy difference. Whenthe electrons are trapped by the trap level, a negative fixed charge isgenerated at the interface with the insulating film, whereby thethreshold voltage of the transistor shifts in the positive direction.Thus, it is preferable that the energy difference between EcS1 and EcS2and the energy difference between EcS1 and EcS3 be 0.1 eV or more,further preferably 0.15 eV or more, because a change in the thresholdvoltage of the transistor is reduced and stable electricalcharacteristics are obtained.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments andexamples.

Embodiment 2

In this embodiment, an oxide semiconductor film included in atransistor, defects included in an oxide insulating film in contact withthe oxide semiconductor film, and the deterioration of transistorcharacteristics are described.

<1. NO_(x)>

First, nitrogen oxide (hereinafter NO_(x); x is greater than or equal to0 and smaller than or equal to 2, preferably greater than or equal to 1and smaller than or equal to 2) contained in the oxide insulating filmin contact with the oxide semiconductor film is described.

<1-1. Transition Level of NO in Oxide Insulating Film>

First, transition levels of point defects in a solid are described. Atransition level shows the charge state of impurities or defects(hereinafter referred to as a defect D) forming a state in a gap, and iscalculated from the formation energy of defects. In other words, atransition level is similar to a donor level or an acceptor level.

The relation between formation energy and transition levels of thecharge state of the defect D and is described. The formation energy ofthe defect D is different depending on the charge state and also dependson the Fermi energy. Note that D⁺ represents a state in which a defectreleases one electron, D⁻ represents a state in which a defect traps oneelectron, and D⁰ represents a state in which no electron is transferred.

FIG. 17A illustrates the relation between the formation energy and thetransition level of each of the defects D⁺, D⁰, and D⁻. FIG. 17Billustrates electron configurations of the defects D⁺, D⁰, and D⁻ in thecase where the defect D in a neutral state has an orbit occupied by oneelectron.

In FIG. 17A, the dotted line indicates the formation energy of thedefect D⁺, the solid line indicates the formation energy of the defectD⁰, and the dashed line indicates the formation energy of the defect D⁻.The transition level means the position of the Fermi level at which theformation energies of the defects D having different charge statesbecome equal to each other. The position of the Fermi level at which theformation energy of the defect D⁺ becomes equal to that of the defect D⁰(that is, a position at which the dotted line and the solid lineintersect) is denoted by ∈(+/0), and the position of the Fermi level atwhich the formation energy of the defect D⁰ becomes equal to that of thedefect D⁻ (that is, a position at which the solid line and the dashedline intersect) is denoted by ∈(0/−).

FIG. 18 illustrates a conceptual diagram of transition of charge statesof a defect that are energetically stable when the Fermi level ischanged. In FIG. 18, the dashed double-dotted line indicates the Fermilevel. Right views of FIG. 18 are band diagrams of (1), (2), and (3)that indicate the Fermi level in a left view of FIG. 18.

By finding out the transition level of a solid, it is qualitativelyknown that which charge state allows a detect to be energetically stableat each of the Fermi levels when the Fermi level is used as a parameter.

As a typical example of the oxide insulating film in contact with theoxide semiconductor film, a silicon oxynitride (SiON) film was used, andthe defect level in the silicon oxynitride film and an ESR signalattributed to the defect level were examined by calculation.Specifically, models in which a nitrogen dioxide molecule, a dinitrogenmonoxide molecule, a nitrogen monoxide molecule, and a nitrogen atomwere introduced into the respective silicon oxide (SiO₂) were formed,and the transition levels thereof were examined to verify whether theatoms introduced into silicon oxide serve as electron traps of thetransistor.

In calculation, SiO₂ (c-SiO₂) with a low-temperature quartz (α-quartz)crystal structure was used as a model. A crystal model of c-SiO₂ withoutdefects is shown in FIG. 8.

First, structure optimization calculation was performed on a modelincluding 72 atoms, particularly on the lattice constants and the atomiccoordinates. The model was obtained by doubling the unit cells in allaxis direction of c-SiO₂. In the calculation, first principlescalculation software VASP (the Vienna Ab initio Simulation Package) wasused. The effect of inner-shell electron was calculated by a projectoraugmented wave (PAW) method, and as a functional,Heyd-Scuseria-Ernzerhof (HSE) DFT hybrid factor (HSE06) was used. Thecalculation conditions are shown below.

TABLE 1 Software VASP Pseudopotential PAW method Functional HSE06 Mixingratio of exchange term 0.4 Cut-off energy 800 eV k-point 1 × 1 × 1(optimization) 2 × 2 × 2 (total energy)

The band gap of c-SiO₂ model after the structure optimization was 8.97eV that is close to the experimental value, 9.0 eV.

Next, the structure optimization calculation was performed on the abovec-SiO₂ models where a nitrogen dioxide molecule, a dinitrogen monoxidemolecule, a nitrogen monoxide molecule, and a nitrogen atom wereintroduced into spaces (interstitial sites) in respective crystalstructures. The structure optimization calculation was performed on eachmodel with respect to the following three cases: a case where the wholemodel is positive monovalent (charge: +1); a case where the whole modelis electrically neutral (zerovalent) (charge: neutral); and a case wherethe whole model is negative monovalent (charge: −1). Note that thecharges imposed on the whole model, which were in the ground state ofelectrons, were localized in defects including the nitrogen dioxidemolecule, the dinitrogen monoxide molecule, the nitrogen monoxidemolecule, and the nitrogen atom.

As for the model in which a nitrogen dioxide molecule was introducedinto an interstitial site in the c-SiO₂ model, a structure after thestructure optimization calculation was performed and structuralparameters of the nitrogen dioxide molecule are shown in FIG. 9. In FIG.9, structural parameters of the nitrogen dioxide molecule in a gaseousstate are also shown as a reference example.

Note that the molecule that is not electrically neutral is frequentlycalled a molecular ion; however, unlike a gaseous state, it is difficultto quantitate the valence of molecule because the molecular discussedhere is one introduced inside a crystal lattice. Thus, a molecule thatis not electrically neutral is called molecular for convenience.

FIG. 9 shows that when the nitrogen dioxide molecule is introduced, thenitrogen dioxide molecule tends to be in a linear arrangement in thecase where the charge of the model is +1. FIG. 9 also shows that theangle of the O—N—O bond of the model whose charge is −1 is smaller thanthat of the model whose charge is neutral, and the angle of the O—N—Obond of the model whose charge is neutral is smaller than that of themodel whose charge is +1. This structure change in the nitrogen dioxidemolecule is almost equal to a change in the bonding angle when thecharge number of isolated molecules in a gas phase varies. Thus, it issuggested that almost the assumed charges are attributed to the nitrogendioxide molecule, and the nitrogen dioxide molecule in SiO₂ probablyexists in a state close to an isolated molecule.

Next, as for the model in which a dinitrogen monoxide molecule wasintroduced into an interstitial site in the c-SiO₂ model, a structureafter the structure optimization calculation was performed andstructural parameters of the dinitrogen monoxide molecule are shown inFIG. 10. In FIG. 10, structural parameters of the dinitrogen monoxidemolecule in a gaseous state are also shown as a reference example.

According to FIG. 10, in the case where the charge of the model is +1and the case where the charge is neutral, the structures of thedinitrogen monoxide molecules are both in a linear arrangement, whichmeans the dinitrogen monoxide molecules of two cases have almost thesame structure. In contrast, in the case where the charge of the modelis −1, the dinitrogen monoxide molecule has a bent shape, and thedistance between the nitrogen atom and the oxygen atom is longer thanthose of the above two cases. This conceivable reason is that anelectron enters the LUMO level that is π* orbital of the dinitrogenmonoxide molecule.

Next, as for the model in which a nitrogen monoxide molecule wasintroduced into an interstitial site in the c-SiO₂ model, a structureafter the structure optimization calculation was performed andstructural parameters of the nitrogen monoxide molecule are shown inFIG. 11.

According to FIG. 11, the distance between a nitrogen atom and an oxygenatom is short in the case where the charge of the model is +1, and thedistance between a nitrogen atom and an oxygen atom is long in the casewhere the charge of the model is −1. This tendency is probably caused bythe following reason. In the case where the charge of the nitrogenmonoxide molecule in a gaseous state is +1, the bond order of the N—Obond is 3.0; in the case where the charge of the nitrogen monoxidemolecule in a gaseous state is 0, the bond order is 2.5; and in the casewhere the charge of the nitrogen monoxide molecule in a gaseous state is−1, the bond order is 2.0. Thus, the bond order becomes the largest whenthe charge is +1. Therefore, the nitrogen monoxide molecule in SiO₂ isconsidered to exist stably in a state close to the isolated molecule.

Then, as for the model in which a nitrogen atom was introduced into aninterstitial site in the c-SiO₂ model, a structure after the structureoptimization calculation was performed is shown in FIG. 12.

According to FIG. 12, in either charge state, the nitrogen atom that isbonded to atoms in SiO₂ is more stable in terms of energy than thenitrogen atom exists as an isolated atom in an interstitial site.

Next, the calculation of a transition level was performed on eachsample.

The transition level ∈(q/q′) for transition between the charge q stateand the charge q′ state in a model having defect D in its structure canbe calculated with Formula 1.

$\begin{matrix}{{{ɛ\left( {q/q^{\prime}} \right)} = \frac{{\Delta \; E^{q}} - {\Delta \; E^{q^{\prime}}}}{q^{\prime} - q}}{{\Delta \; E^{q}} = {{E_{tot}\left( D^{q} \right)} - {E_{tot}({bulk})} + {\sum\limits_{i}{n_{i}\mu_{i}}} + {q\left( {ɛ_{VBM} + {\Delta \; V_{q}} + E_{F}} \right)}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In the above formula, E_(tot) (D^(q)) represents the total energy in themodel having defect D of the charge q, E_(tot) (bulk) represents thetotal energy in a model without defects, n_(i) represents the number ofatoms i contributing to defects, μ_(i) represents the chemical potentialof atom i, ∈_(VBM) represents the energy of the valence band maximum inthe model without defects, ΔV_(q) represents the correction termrelating to the electrostatic potential, and E_(F) represents Fermienergy.

FIG. 13 is a band diagram showing the transition levels obtained fromthe above formula. As the oxide semiconductor film, an oxidesemiconductor film (hereinafter referred to as IGZO(111)) formed usingmetal oxide having an atomic ratio of In:Ga:Zn=1:1:1 is used. In FIG.13, a band diagram of the IGZO(111) is shown in addition to the banddiagrams of the above four models. The unit of the values in FIG. 13 is“eV”.

In FIG. 13, the value of each transition level indicates a valueobtained when the valence band maximum of SiO₂ is considered as a base(0.0 eV). Although a reference value was used as an electron affinity ofSiO₂ here, the practical positional relation of the bands in the casewhere SiO₂ is bonded to the IGZO(111) is affected by the electronaffinity of SiO₂ in some cases.

Hereinafter, the transition level that transits between a state wherethe charge of the model is +1 and a state where the charge of the modelis 0 is referred to as (+/0), and the transition level that transitsbetween a state where the charge of the model is 0 and a state where thecharge of system is −1 is referred to as (0/−).

According to FIG. 13, in the model in which the nitrogen dioxidemolecule was introduced into SiO₂, two transition levels of (+10) and(0/−) exist at the positions within the band gap of the IGZO(111), whichsuggests that the nitrogen dioxide molecule may relate to trap anddetrap of electrons. In both a model in which a nitrogen monoxidemolecule was introduced into SiO₂ and a model in which a nitrogen atomwas introduced into SiO₂, the transition level of (+/0) exits at aposition within the band gap of the IGZO(111). In contrast, thetransition level of the model in which a dinitrogen monoxide moleculewas introduced into SiO₂ exists outside of the band gap of theIGZO(111), and the dinitrogen monoxide molecules probably exist stablyas neutral molecules regardless of the position on Fermi level.

The above results strongly suggest that interstitial moleculescontaining nitrogen, which relate to trap and detrap of electrons causedby a shift of the threshold voltage of a transistor in the positivedirection, have the transition level at a position on the conductionband side within the band gap of the IGZO(111). Here, a molecule havinga transition level at a position close to the conduction band in theband gap of the IGZO(111) is probably a nitrogen dioxide molecule or anitrogen monoxide molecule, or both.

<1-2. Examination of ESR Signal>

Following the calculation results of the transition level, ESR signalsof nitrogen dioxide molecules were calculated. In addition, a model inwhich a nitrogen atom substituted for an oxygen atom in SiO₂ wasexamined in a manner similar to that of the above case.

In this case, a nitrogen atom has seven electrons, and an oxygen atomhas eight electrons; in other words, an electron structure of thenitrogen dioxide molecule has an open shell. Thus, the neutral nitrogendioxide molecule has a lone electron, and can be measured by ESR. In thecase where a nitrogen atom substitutes for an oxygen atom in SiO₂, onlytwo silicon atoms exist around a nitrogen atom, and the nitrogen atomincludes a dangling bond. Thus, the case can also be measured by ESR.Furthermore, ¹⁴N has only one nuclear spin, and a peak of ESR signalrelating to ¹⁴N is split into three. At this time, the split width ofESR signal is a hyperfine coupling constant.

Thus, calculation was performed to examine whether split of an ESRsignal of the oxide insulating film into three is caused by the nitrogendioxide molecule or the nitrogen atom that replaces an oxygen atom inSiO₂. When an SiO₂ crystal structure is used as a model, the amount ofcalculation is enormous. Thus, in this case, two kinds of models ofcluster structures as shown in FIGS. 14A and 14B were used, thestructure optimization was performed on these models, and then,g-factors and hyperfine coupling constants were calculated. FIG. 14Ashows a model of a nitrogen dioxide molecule in a neutral state, andFIG. 14B shows a cluster model including a Si—N—Si bond. Note that themodel shown in FIG. 14B is a cluster model in which a dangling bond of asilicon atom is terminated with a hydrogen atom.

Amsterdam density functional (ADF) software was used for structureoptimization of the models and calculation of the g-factors andhyperfine coupling constants of the models whose structures wereoptimized. In the structure optimization and the calculation of themodels and the g-factors and hyperfine coupling constants of the modelswhose structures were optimized, “GGA:BP” was used as a functional, and“QZ4P” was used as a basic function, and “None” was used as Core Type.In addition, in the calculation of the g-factors and hyperfine couplingconstants, “Spin-Orbit” was considered as a relativistic effect, and asa calculation method of ESR/EPR, “g & A-Tensor (full SO)” was employed.The calculation conditions are as follows.

TABLE 2 Software ADF Basis function QZ4P Functional GGA-BP Core TypeNone Relativistic Effect Spin-Orbit Calculation method of ESR/EPR g &A-Tensor (full SO)

As a result of structure optimization, in the case of the nitrogendioxide molecule shown in FIG. 14A, the bonding distance of the N—O bondwas 0.1205 nm, and the angle of the O—N—O bond was 134.1°, which areclose to experimental values of the nitrogen dioxide molecule (thebonding distance: 0.1197 nm, and the bonding angle) 134.3°. In the caseof the Si—N—Si cluster model shown in FIG. 14B, the bonding distance ofSi—N was 0.172 nm and the angle of the Si—N—Si bond was 138.3°, whichwere equivalent to the bonding distance of Si—N (0.170 nm) and the angleof the Si—N—Si bond (139.0°) in the structure that had been subjected tostructure optimization by first principles calculation in a state wherea nitrogen atom substitutes for an oxygen atom in the SiO₂ crystal.

The calculated g-factors and hyperfine coupling constants are shownbelow.

TABLE 3 Hyperfine coupling g-factor constant [mT] g_x g_y g_z g(average) A_x A_y A_z A (average) NO₂ 2.0066 1.9884 2.0014 1.9988 4.544.49 6.53 5.19 Si—N—Si 2.0021 2.0174 2.0056 2.0084 3.14 −0.61 −0.62 0.64

As described above, the hyperfine coupling constant A corresponds to thesplit width of a peak of the ESR signal. According to Table 3, theaverage value of the hyperfine coupling constant A of the nitrogendioxide molecule is approximately 5 mT. In the case of the Si—N—Sicluster model, only A_x in the hyperfine coupling constants A is apositive value, which is approximately 3 mT.

According to this result, the ESR spectrum that has three signals, ahyperfine structure constant of approximately 5 mT, and a g-factor ofapproximately 2, which are obtained by ESR measurement using an X-band,is obtained probably because of a nitrogen dioxide molecule in an SiO₂crystal. Among three signals, the g-factor of the medium signal isapproximately 2.

<1-3. Consideration of Deterioration Mechanism of Transistor>

A mechanism of a phenomenon in which the threshold voltage of atransistor is shifted in the positive direction when a positive gate BTstress test (+GBT) is performed is considered below based on the aboveresults.

The mechanism is considered with reference to FIG. 15. FIG. 15illustrates a structure in which a gate (GE), a gate insulating film(GI), an oxide semiconductor film (OS), and a silicon oxynitride film(SiON) are stacked in this order. Here, a case where the siliconoxynitride film SiON that is positioned on the back channel side of theoxide semiconductor film (OS) contains nitrogen oxide is described.

First, when the positive gate BT stress test (+GBT) is performed, theelectron densities of the gate insulating film GI side and the siliconoxynitride film SiON side of the oxide semiconductor film OS becomehigher. In the oxide semiconductor film OS, the silicon oxynitride filmSiON side has a lower electron density than the gate insulating film GIside. When a nitrogen dioxide molecule or a nitrogen monoxide moleculecontained in the silicon oxynitride film SiON is diffused into theinterface between the gate insulating film GI and the oxidesemiconductor film OS and the interface between the oxide semiconductorfilm OS and the silicon oxynitride film SiON, electrons on the gateinsulating film GI side and the back channel side that are induced bythe positive gate BT stress test (+GBT) are trapped. As a result, thetrapped electrons remain in the vicinity of the interface between thegate insulating film GI and the oxide semiconductor film OS and theinterface between the oxide semiconductor film OS and the siliconoxynitride film SiON; thus, the threshold voltage of the transistor isshifted in the positive direction.

That is, a lower concentration of nitrogen oxide contained in thesilicon oxynitride film in contact with the oxide semiconductor film cansuppress a change in the threshold voltage of the transistor. Here, asspecific examples of the silicon oxynitride film in contact with theoxide semiconductor film, the protective film in contact with the backchannel side, the gate insulating film, and the like can be given. Byproviding the silicon oxynitride film containing an extremely smallamount of nitrogen oxide in contact with the oxide semiconductor film,the transistor can have excellent reliability.

<2. V_(o)H>

Next, a hydrogen atom (hereinafter referred to as V_(o)H) positioned inan oxygen vacancy V_(o), which is one of defects contained in the oxidesemiconductor film, is described.

<2-1. Energy and Stability Between Existing Modes of H>

First, the energy difference and stability in a mode of H that exists inan oxide semiconductor film is described with calculated results. Here,InGaZnO₄ (hereinafter referred to as IGZO (111)) was used as the oxidesemiconductor film.

The structure used for the calculation is based on a 84-atom bulk modelin which twice the number of a hexagonal unit cell of the IGZO(111) isarranged along the a-axis and b-axis.

As the bulk model, a model in which one oxygen atom bonded to threeindium atoms and one zinc atom is replaced with a hydrogen atom wasprepared (see FIG. 16A). FIG. 16B shows a diagram in which the a-b planeof the InO layer in FIG. 16A is viewed from the c-axis direction. Aregion from which one oxygen atom bonded to three indium atoms and onezinc atom is removed is shown as an oxygen vacancy V_(o), which is shownin a dashed line in FIGS. 16A and 16B. In addition, a hydrogen atom inthe oxygen vacancy V_(o) is expressed as V_(o)H.

In the bulk model, one oxygen atom bonded to three indium atoms and onezinc atom is removed, whereby an oxygen vacancy V_(o) is formed. A modelin which, in the vicinity of the oxygen vacancy V_(o), a hydrogen atomis bonded to one oxygen atom to which one gallium atom and two zincatoms are bonded on the a-b plane was prepared (see FIG. 16C). FIG. 16Dshows a diagram in which the a-b plane of the InO layer in FIG. 16C isviewed from the c-axis direction. In FIGS. 16C and 16D, an oxygenvacancy V_(o) is shown in a dashed line. A model in which an oxygenvacancy V_(o) is formed and, in the vicinity of the oxygen vacancyV_(o), a hydrogen atom is bonded to one oxygen atom to which one galliumatom and two zinc atoms are bonded on the a-b plane is expressed asV_(o)+H.

Optimization calculation was performed on the above two models with afixed lattice constant to calculate the total energy. Note that as thevalue of the total energy is smaller, the structure becomes more stable.

In the calculation, first principles calculation software VASP (TheVienna Ab initio simulation Package) was used. The calculationconditions are shown in Table 4.

TABLE 4 Software VASP Pseudopotential PAW method Functional GGA/PBECut-off energy 500 eV k-point 4 × 4 × 1

As pseudopotential calculation of electronic states, a potentialgenerated by a projector augmented wave (PAW) method was used, and as afunctional, generalized-gradient-approximation/Perdew-Burke-Ernzerhof(GGA/PBE) was used.

In addition, the total energy of the two models that were obtained bythe calculations is shown in Table 5.

TABLE 5 Model Total energy VoH −456.084 eV Vo + H −455.304 eV

According to Table 5, the total energy of V_(o)H is lower than that ofV_(o)+H by 0.78 eV. Thus, V_(o)H is more stable than V_(o)+H. Thissuggests that, when a hydrogen atom comes close to an oxygen vacancy(V_(o)), the hydrogen atom is easily trapped in the oxygen vacancy(V_(o)) than bonding with an oxygen atom.

<2-2. Thermodynamic State of V_(o)H>

Next, the thermodynamic state of V_(o)H, which is a hydrogen atomtrapped in an oxygen vacancy (V_(o)), is evaluated with electronic statecalculation, and the results are described.

The formation energies of the defects V_(o)H contained in the IGZO(111),(V_(o)H)⁺, (V_(o)H)⁻, and (V_(o)H)⁰, were calculated. Note that(V_(o)H)⁺ represents a state in which a defect releases one electron,(V_(o)H)⁻ represents a state in which a defect traps one electron, and(V_(o)H)⁰ represents a state in which no electron is transferred.

In the calculation, the first principles calculation software VASP wasused. The calculation conditions are shown in Table 6. FIG. 19illustrates a model that were used for the calculation. The formationenergy was calculated on the assumption of the reaction in Formula 2. Aspseudopotential calculation of electronic states, a potential generatedby a PAW method was used, and as a functional, Heyd-Scuseria-Ernzerhof(HSE) DFT hybrid factor (HSE06) was used. Note that the formation energyof an oxygen vacancy was calculated as follows: a dilute limit of theconcentration of oxygen vacancies was assumed, and excessive expansionof electrons and holes to the conduction band and the valence band wascorrected. In addition, shift of the valence band due to the defectstructure was corrected using the average electrostatic potential withthe valence band maximum of a complete crystal serving as the origin ofenergy.

TABLE 6 Software VASP Pseudopotential PAW method Functional HSE06Cut-off energy 800 eV The number of K-point samples 2 × 2 × 1(optimization) 4 × 4 × 1 (single) Spin Polarized Shielding parameter 0.2 Exchange term mixing ratio  0.25 The number of atoms 84

$\begin{matrix}{\left. {IGZO}\rightarrow{{{IGZO}\text{:}\mspace{14mu} {{Vo}H}} + {\frac{1}{2}O_{2}} - {\frac{1}{2}H_{2}}} \right.{{E_{form}\left( {{IGZO}\text{:}{{Vo}H}} \right)} = {{E_{tot}\left( {{IGZO}\text{:}{{Vo}H}} \right)} - {E_{tot}({IGZO})} + {\frac{1}{2}{E_{tot}\left( O_{2} \right)}} - {\frac{1}{2}{E_{tot}\left( H_{2} \right)}}}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

The formation energy obtained by the calculation is shown in FIG. 20A.

FIG. 20A shows the formation energies of (V_(o)H)⁺, (V_(o)H)⁻, and(V_(o)H)⁰. The horizontal axis represents the Fermi level, and thevertical axis represents the formation energy. The dotted linerepresents the formation energy of (V_(o)H)⁺, the solid line representsthe formation energy of (V_(o)H)⁰, and the dashed line represents theformation energy of (V_(o)H)⁻. In addition, the transition level of theV_(o)H charge from (V_(o)H)⁺ to (V_(o)H)⁻ through (V_(o)H)⁰ isrepresented by ∈(+/−).

FIG. 20B shows a thermodynamic transition level of V_(o)H. From thecalculation result, the energy gap of InGaZnO₄ was 2.739 eV. Inaddition, when the energy of the valence band is 0 eV, the transferlevel (∈(+/−)) is 2.62 eV, which exists just under the conduction band.These suggest that in the case where the Fermi level exists in theenergy gap, the charge state of V_(o)H is always +1 and V_(o)H serves asa donor. This shows that IGZO(111) becomes n-type by trapping a hydrogenatom in an oxygen vacancy (V_(o)).

Next, FIG. 21 shows the results of evaluation of the relation betweenthe carrier (electron) density and the defect (V_(o)H) density.

FIG. 21 shows that the carrier density increases as the defect (V_(o)H)density increases.

Accordingly, it is found that V_(o)H in the IGZO(111) serves as a donor.In addition, it is also found that when the density of V_(o)H becomeshigh, the IGZO(111) becomes n-type.

<3. Model Explaining Relation Between DOS in Oxide Semiconductor Filmand Element to be DOS>

When density of states (DOS) exists inside an oxide semiconductor filmand in the vicinity of the interface between the oxide semiconductorfilm and the outside, DOS can cause deterioration of a transistorincluding the oxide semiconductor film. The DOS inside the oxidesemiconductor film and in the vicinity of the interface with the oxidesemiconductor film can be explained on the basis of the positions of andthe bonding relation among oxygen (O), an oxygen vacancy (V_(o)),hydrogen (H), and nitrogen oxide (NO_(x)). A concept of a model isdescribed below.

In order to fabricate a transistor with stable electricalcharacteristics, it is important to reduce the DOS inside the oxidesemiconductor film and in the vicinity of the interface (to make ahighly purified intrinsic state). In order to reduce the DOS, oxygenvacancies, hydrogen, and nitrogen oxide should be reduced.

FIG. 22 illustrates a band structure of DOS inside an oxidesemiconductor film and in the vicinity of the interface of the oxidesemiconductor film. The case where the oxide semiconductor film is theoxide semiconductor film (IGZO(111)) containing indium, gallium, andzinc is described below.

There are two types of DOS, DOS at a shallow level (shallow level DOS)and DOS at a deep level (deep level DOS). Note that in thisspecification, the shallow level DOS refers to DOS between energy at theconduction band minimum (Ec) and the mid gap. Thus, for example, theshallow level DOS is located closer to energy at the conduction bandminimum. Note that in this specification, the deep level DOS refers toDOS between energy at the valence band maximum (Ev) and the mid gap.Thus, for example, the deep level DOS is located closer to the mid gapthan to energy at the valence band maximum.

In the oxide semiconductor film, there are two types of shallow levelDOS. One is DOS in the vicinity of a surface of an oxide semiconductorfilm (at the interface with an insulating film (insulator) or in thevicinity of the interface with the insulating film), that is, surfaceshallow DOS. The other is DOS inside the oxide semiconductor film, thatis, bulk shallow DOS. Furthermore, as a type of the deep level DOS,there is DOS inside the oxide semiconductor film, that is, bulk deepDOS.

These types of DOS are likely to act as described below. The surfaceshallow DOS in the vicinity of the surface of an oxide semiconductorfilm is located at a shallow level from the conduction band minimum, andthus trap and loss of an electric charge are likely to occur easily inthe surface shallow DOS. The bulk shallow DOS inside the oxidesemiconductor film is located at a deep level from the conduction bandminimum as compared to the surface shallow DOS in the vicinity of thesurface of the oxide semiconductor film, and thus loss of an electriccharge does not easily occur in the bulk shallow DOS.

An element causing DOS in an oxide semiconductor film is describedbelow.

For example, when a silicon oxide film is formed over an oxidesemiconductor film, indium contained in the oxide semiconductor film istaken into the silicon oxide film and replaces silicon to form shallowlevel DOS.

For example, in the interface between the oxide semiconductor film andthe silicon oxide film, a bond between oxygen and indium contained inthe oxide semiconductor film is broken and a bond between the oxygen andsilicon is generated. This is because the bonding energy between siliconand oxygen is higher than the bonding energy between indium and oxygen,and the valence of silicon (tetravalence) is larger than the valence ofindium (trivalence). Oxygen contained in the oxide semiconductor film istrapped by silicon, so that a site of oxygen that has been bonded toindium becomes an oxygen vacancy. In addition, this phenomenon occurssimilarly when silicon is contained inside the oxide semiconductor film,as well as in the surface. Such an oxygen vacancy forms deep level DOS.

Another cause as well as silicon can break the bond between indium andoxygen. For example, in an oxide semiconductor film containing indium,gallium, and zinc, the bond between indium and oxygen is weaker and cutmore easily than the bond between oxygen and gallium or zinc. For thisreason, the bond between indium and oxygen is broken by plasma damagesor damages due to sputtered particles, so that an oxygen vacancy can beproduced. The oxygen vacancy forms deep level DOS.

The deep level DOS can trap a hole and thus serve as a hole trap (holetrapping center). This means that the oxygen vacancy forms bulk deep DOSinside the oxide semiconductor film. Since such an oxygen vacancy formsbulk deep DOS, the oxygen vacancy is an instability factor to the oxidesemiconductor film.

Such deep level DOS due to an oxygen vacancy is one of causes forforming bulk shallow DOS in the oxide semiconductor film, which isdescribed below.

In addition, an oxygen vacancy in the oxide semiconductor film trapshydrogen to be metastable. That is, when an oxygen vacancy that is deeplevel DOS and is capable of trapping a hole traps hydrogen, the oxygenvacancy forms bulk shallow DOS and becomes metastable. As described in<Thermodynamic state of V_(o)H> of this embodiment, when an oxygenvacancy traps hydrogen, the oxygen vacancy is neutrally or positivelycharged. That is, V_(o)H, which is one bulk shallow DOS in the oxidesemiconductor film, releases an electron, to be neutrally or positivelycharged, which adversely affects the characteristics of a transistor.

It is important to reduce the density of oxygen vacancies to prevent anadverse effect on the characteristics of the transistor. Thus, bysupplying excess oxygen to the oxide semiconductor film, that is, byfilling oxygen vacancies with excess oxygen, the density of oxygenvacancies in the oxide semiconductor film can be lowered. In otherwords, the oxygen vacancies become stable by receiving excess oxygen.For example, when excess oxygen is included in the oxide semiconductorfilm or an insulating film provided near the interface with the oxidesemiconductor film, the excess oxygen can fill oxygen vacancies in theoxide semiconductor film, thereby effectively eliminating or reducingoxygen vacancies in the oxide semiconductor film.

As described above, the oxygen vacancy may become a metastable state ora stable state by hydrogen or oxygen.

As described in <Transition level of NO in oxide insulating film> ofthis embodiment, nitrogen monoxide or nitrogen dioxide, which is NO_(x),traps an electron included in the oxide semiconductor film. Becausenitrogen monoxide or nitrogen dioxide, which is NO_(x), is surfaceshallow DOS in the vicinity of the surface of the oxide semiconductorfilm, when NO_(x) is included in the insulating film in the vicinity ofthe interface with the oxide semiconductor film, the characteristics ofa transistor are adversely affected.

It is important to reduce the content of NO_(x) in the insulating filmin the vicinity of the interface with the oxide semiconductor film toprevent an adverse effect on the characteristics of the transistor.

<3-1. Model of Hysteresis Deterioration in Dark State of TransistorIncluding Oxide Semiconductor Film>

A mechanism in deterioration of a transistor including an oxidesemiconductor film is described next. The transistor including an oxidesemiconductor film deteriorates differently depending on whether or notthe transistor is irradiated with light. When the transistor isirradiated with light, deterioration is likely to result from the bulkdeep DOS at the deep level inside the oxide semiconductor film. When thetransistor is not irradiated with light, deterioration is likely toresult from the surface shallow DOS at the shallow level in the vicinityof the surface of the oxide semiconductor film (at the interface with aninsulating film or in the vicinity thereof).

Thus, a state where the transistor including an oxide semiconductor filmis not irradiated with light (dark state) is described. In the darkstate, the deterioration mechanism of the transistor can be explained onthe basis of trapping and releasing of a charge by the surface shallowDOS at the shallow level in the vicinity of the surface of the oxidesemiconductor film (at the interface with an insulating film or in thevicinity of the interface). Note that here, a gate insulating film isdescribed as an insulating film provided in the vicinity of theinterface with the oxide semiconductor film.

FIG. 23 shows variation in a threshold voltage (V_(th)) when thetransistor including an oxide semiconductor film is subjected to a gatebias temperature (BT) stress test repeatedly in the dark state. Asapparent from FIG. 23, the threshold voltage is shifted to a positiveside by the positive gate BT (+GBT) stress test. Then, the transistor issubjected to a negative gate BT (−GBT) stress test, so that thethreshold voltage is shifted to a negative side and is substantiallyequal to the initial value (initial). In this manner, by repeating thepositive gate BT stress test and the negative gate BT stress testalternately, the threshold voltage is shifted positively and negatively(i.e., a hysteresis occurs). In other words, it is found that when thepositive gate BT stress test and the negative gate BT stress test arerepeated without light irradiation, the threshold voltage is shiftedalternately to a positive side and then a negative side, but the shiftfits in certain range as a whole.

The variation in the threshold voltage of the transistor due to the gateBT stress test in the dark state can be explained with the surfaceshallow DOS in the vicinity of the surface of an oxide semiconductorfilm. FIG. 24 illustrates a band structure of an oxide semiconductorfilm and flow charts corresponding to the band structure.

Before application of the gate BT stress (at the gate voltage (V_(g)) of0), the surface shallow DOS in the vicinity of the surface of an oxidesemiconductor film has energy higher than the Fermi level (E_(f)) and iselectrically neutral since an electron is not trapped (Step S101 in FIG.24). In Step S101, the threshold voltage measured at this time is set asan initial value before the gate BT stress is applied.

Next, the positive gate BT stress test (dark state) is performed. Whenthe positive gate voltage is applied, the conduction band is curved andthe energy of the surface shallow DOS in the vicinity of the surface ofthe oxide semiconductor film becomes lower than the Fermi level. Thus,an electron is trapped in the surface shallow DOS in the vicinity of thesurface of the oxide semiconductor film, so that the DOS is chargednegatively (Step S102 in FIG. 24).

Next, the application of stress is stopped such that the gate voltage is0. By the gate voltage at 0, the surface shallow DOS in the vicinity ofthe surface of an oxide semiconductor film has energy higher than theFermi level. However, it takes a long time for the electron trapped inthe surface shallow DOS in the vicinity of the surface of the oxidesemiconductor film to be released. Thus, the surface shallow DOS in thevicinity of the surface of the oxide semiconductor film remains chargednegatively (Step S103 in FIG. 24). At this time, a channel formationregion of the transistor is being subjected to application of a negativevoltage as well as the gate voltage. Accordingly, a gate voltage that ishigher than the initial value should be applied so as to turn on thetransistor, so that the threshold voltage is shifted to a positive side.In other words, the transistor tends to be normally off.

Next, a negative gate voltage is applied as the negative gate BT stresstest (dark state). When the negative gate voltage is applied, theconduction band is curved and the energy of the surface shallow DOS inthe vicinity of the surface of the oxide semiconductor film becomes muchhigher. Thus, the electron trapped in the surface shallow DOS in thevicinity of the surface of the oxide semiconductor film is released, sothat the DOS becomes electrically neutral (Step S104 in FIG. 24).

Next, the application of stress is stopped such that the gate voltage is0. The surface shallow DOS in the vicinity of the surface of an oxidesemiconductor film at this time has released the electron and iselectrically neutral (Step S101). Thus, the threshold voltage is shiftedto a positive side, so that it returns to the initial value before thegate BT stress tests. The negative gate BT test and the positive gate BTstress test are repeated in the dark state, so that the thresholdvoltage is shifted repeatedly to the positive side and to the negativeside. However, an electron trapped in the surface shallow DOS in thevicinity of the surface of an oxide semiconductor film at the time ofthe positive gate BT stress test is released at the time of the negativegate BT stress test; therefore, it is found that the threshold voltageis shifted within a certain range as a whole.

As described above, the shift in the threshold voltage of a transistordue to the gate BT stress test in the dark state can be explained on thebasis of the understanding of the surface shallow DOS in the vicinity ofthe surface of the oxide semiconductor film.

<3-2. Model of Deterioration in Bright State of Transistor IncludingOxide Semiconductor Film>

Then, a deterioration mechanism under light irradiation (bright state)is described here. The deterioration mechanism of the transistor in thebright state is explained on the basis of the trap and release of anelectron in the bulk deep DOS at the deep level in the oxidesemiconductor film.

FIG. 25 shows the shift in the threshold voltage (V_(th)) when the gateBT stress test is conducted repeatedly on the transistor including anoxide semiconductor film in the bright state. As shown in FIG. 25, thethreshold voltage (V_(th)) is shifted from the initial value (initial)in the negative direction.

In FIG. 25, a value measured in the dark state without application ofthe gate BT stress is plotted as the initial value of the thresholdvoltage. Then, the threshold voltage is measured in the bright statewithout application of the gate BT stress. As a result, the thresholdvoltage in the bright state is shifted to a negative side greatly fromthe threshold voltage in the dark state. One of the conceivable factorsis that an electron and a hole are generated by light irradiation andthe generated electron is excited to the conduction band. In otherwords, even when the gate BT stress is not applied, the thresholdvoltage of the transistor including an oxide semiconductor film isshifted to a negative side by light irradiation, so that the transistoris easily normally on. In this case, as the energy gap of the oxidesemiconductor film is larger, or as fewer DOS exist in the gap, fewerelectrons are excited. For that reason, the shift in the thresholdvoltage due to light irradiation is small in that case.

Then when the negative gate BT stress is applied under light irradiation(−GBT), the threshold voltage is further shifted to a negative side.

After that, the positive gate BT (+GBT) stress test is conducted underlight irradiation, so that the threshold voltage is shifted to apositive side.

Further, when the negative gate BT stress test and the positive gate BTstress test are repeated under light irradiation, the threshold voltageis shifted to a positive side and a negative side repeatedly; as aresult, it is found that the threshold voltage is shifted gradually to anegative side as a whole.

In the gate BT stress tests (where the positive gate BT stress test andthe negative gate BT stress test are repeated) in the bright state, amechanism of the shift in the threshold voltage of the transistor isexplained with reference to the band structures in FIG. 26 and FIG. 27.With reference to FIG. 26 and FIG. 27, the bulk deep DOS in the oxidesemiconductor film and the non bridging oxygen hole centers (NBOHC1 andNBOHC2) in the gate insulating film are described. Note that the nonbridging oxygen hole center (NBOHC1) is NBOHC that is located closer tothe interface with the oxide semiconductor film (on the surface side)than the non bridging oxygen hole center (NBOHC2) is.

Before the gate BT stress test and light irradiation (when the gatevoltage (V_(g)) is 0), the bulk deep DOS in the oxide semiconductor filmhas energy lower than the Fermi level (E_(f)), and is electricallyneutral since holes are not trapped (Step S111 in FIG. 26). At thistime, the threshold voltage measured in the dark state is regarded asthe initial value in the dark state.

Next, the oxide semiconductor film is irradiated with light withoutbeing subjected to the gate BT stress, so that electrons and holes aregenerated (Step S112 in FIG. 26). The generated electrons are excited tothe conduction band, so that the threshold voltage is shifted to anegative side (electrons are not described in the subsequent steps). Inaddition, the generated holes lower the quasi-the Fermi level (E_(fp))of holes. Because the quasi-the Fermi level (E_(fp)) of holes islowered, holes are trapped in the bulk deep DOS inside the oxidesemiconductor film (Step S113 in FIG. 26). Accordingly, under lightirradiation without the gate BT stress test, the threshold voltage isshifted to the negative side, so that the transistor easily becomesnormally on, unlike the transistor in the dark state.

Next, the negative gate BT stress test is conducted under lightirradiation, so that an electric field gradient is generated and holestrapped in the bulk deep DOS inside the oxide semiconductor film areinjected to the non bridging oxygen hole center (NBOHC1) in the gateinsulating film (Step S114 in FIG. 26). In addition, some holes moveinto the non bridging oxygen hole centers (NBOHC2) further inside thegate insulating film by the electric field (Step S115 in FIG. 27). Themovement of holes from the non bridging oxygen hole centers (NBOHC1) tothe non bridging oxygen hole centers (NBOHC2) in the gate insulatingfilm progresses with time of the electric field application. The holesin the non bridging oxygen hole centers (NBOHC1 and NBOHC2) in the gateinsulating film act as positively-charged fixed charges, and shift thethreshold voltage to the negative side, so that the transistor easilybecomes normally on.

Light irradiation and the negative gate BT stress test are described asdifferent steps for easy understanding, but the present invention is notconstrued as being limited to description in this embodiment. Forexample, Step S112 to Step S115 can occur in parallel.

Next, the positive gate BT stress test is conducted under lightirradiation, and holes trapped in the bulk deep DOS inside the oxidesemiconductor film and holes in the non bridging oxygen hole centers(NBOHC1) in the gate insulating film are released by the application ofthe positive gate voltage (Step S116 in FIG. 27). Thus, the thresholdvoltage is shifted to the positive side. Note that because the nonbridging oxygen hole center (NBOHC2) in the gate insulating film is atthe deep level in the gate insulating film, almost no holes in the nonbridging oxygen hole centers (NBOHC2) are directly released even whenthe positive gate BT stress test is in the bright state. In order thatthe holes in the non bridging oxygen hole center (NBOHC2) in the gateinsulating film can be released, the holes should move to the nonbridging oxygen hole centers (NBOHC1) on the surface side. The movementof a hole from the non bridging oxygen hole center (NBOHC2) to the nonbridging oxygen hole center (NBOHC1) in the gate insulating filmprogresses little by little with the time of electric field application.Therefore, the shift amount to the positive side of the thresholdvoltage is small, and the threshold voltage does not return completelyto the initial value.

In addition, the movement of a hole occurs between the non bridgingoxygen hole center (NBOHC1) in the gate insulating film and the bulkdeep DOS inside the oxide semiconductor film. However, because manyholes have been trapped in the bulk deep DOS inside the oxidesemiconductor film, the whole electric charge amount of the oxidesemiconductor film and the gate insulating film can be hardly reduced.

Next, the negative gate BT stress test is conducted again under lightirradiation, so that an electric field gradient occurs and holes trappedin the bulk deep DOS inside the oxide semiconductor film are injectedinto the non bridging oxygen hole center (NBOHC1) in the gate insulatingfilm. In addition, some of the holes are injected into the non bridgingoxygen hole center (NBOHC2) that is deeper inside the gate insulatingfilm by an electric field (Step S117 in FIG. 27). Note that the holes inthe non bridging oxygen hole centers (NBOHC2) in the gate insulatingfilm, which have been injected thereinto in Step S115, are left withoutbeing released. Thus, holes are further injected, so that the number ofholes serving as fixed charges is further increased. The thresholdvoltage is further shifted to the negative side, so that the transistorfurther easily becomes normally on.

Next, the positive gate BT stress test is conducted under lightirradiation, so that holes trapped in the bulk deep DOS in the oxidesemiconductor film and holes in the non bridging oxygen hole center(NBOHC1) in the gate insulating film are released by application of thepositive gate voltage (Step S118 in FIG. 27). As a result, the thresholdvoltage is shifted to the positive side. However, the holes in the nonbridging oxygen hole center (NBOHC2) in the gate insulating film arehardly released. Accordingly, the shift amount to the positive side ofthe threshold voltage is small, and the threshold voltage does notreturn completely to the initial value.

It is presumed that by repeating the negative gate BT stress test andthe positive gate BT stress test in the bright state as described above,the threshold voltage is gradually shifted to the negative side as awhole while the threshold voltage is shifted to the positive side andthe negative side repeatedly.

The shift of the threshold voltage of the transistor in the gate BTstress test in the bright state can be explained on the basis of thebulk deep DOS inside the oxide semiconductor film and the non bridgingoxygen hole centers (NBOHC1 and NBOHC2) in the gate insulating film.

<3-3. Process Model of Dehydration, Dehydrogenation, and Oxygen Additionof Oxide Semiconductor Film>

In order to fabricate a transistor with stable electricalcharacteristics, it is important to reduce the DOS inside the oxidesemiconductor film and in the vicinity of the interface of the oxidesemiconductor film (to make a highly purified intrinsic state). Aprocess model where the oxide semiconductor film is highly purified tobe intrinsic is described below. Dehydration or dehydrogenation of theoxide semiconductor film are described first and then oxygen additionwhere an oxygen vacancy (Vo) is filled with oxygen is described.

Before a process model where the oxide semiconductor film is highlypurified to be intrinsic is described, the position at which an oxygenvacancy is likely to be generated in the oxide semiconductor film isdescribed. In the oxide semiconductor film containing indium, gallium,and zinc, the bond between indium and oxygen is broken most easily ascompared to the bond between gallium and oxygen and the bond betweenzinc and oxygen. Thus, a model where the bond between indium and oxygenis broken to form an oxygen vacancy is described below.

When the bond between indium and oxygen is broken, oxygen is releasedand a site of the oxygen that has been bonded to indium serves as anoxygen vacancy. The oxygen vacancy forms the deep level DOS at the deeplevel of the oxide semiconductor film. Because the oxygen vacancy in theoxide semiconductor film is instable, it traps oxygen or hydrogen to bestable. For this reason, when hydrogen exists near an oxygen vacancy,the oxygen vacancy traps hydrogen to become V_(o)H. The V_(o)H forms theshallow level DOS at the shallow level in the oxide semiconductor film.

Next, when oxygen comes close to the V_(o)H in the oxide semiconductorfilm, oxygen extracts hydrogen from V_(o)H to become a hydroxyl group(OH), so that hydrogen is released from the V_(o)H (see FIGS. 28A and28B). The oxygen can move in the oxide semiconductor film so as to comecloser to hydrogen by heat treatment and the like.

Further, when the hydroxyl group comes closer to another V_(o)H in theoxide semiconductor film, the hydroxyl group extracts hydrogen fromV_(o)H to become a water molecule (H₂O), so that hydrogen is releasedfrom V_(o)H (see FIGS. 28C and 28D). In this manner, one oxygen releasestwo hydrogen from the oxide semiconductor film. This is referred to asdehydration or dehydrogenation of the oxide semiconductor film. By thedehydration or dehydrogenation, the shallow level DOS at the shallowlevel in the oxide semiconductor film is reduced, and the deep level DOSis formed.

Next, when oxygen comes close to an oxygen vacancy in the oxidesemiconductor film, oxygen is trapped by the oxygen vacancy, so that theoxygen vacancy is reduced (see FIGS. 28E and 28F). This is referred toas oxygen addition in the oxide semiconductor film. By the oxygenaddition, the deep level DOS at the deep level in the oxidesemiconductor film is reduced.

As described above, when dehydration or dehydrogenation and oxygenaddition of the oxide semiconductor film are performed, the shallowlevel DOS and the deep level DOS in the oxide semiconductor film can bereduced. This process is referred to as a highly purification processfor making an intrinsic oxide semiconductor.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments andexamples.

Embodiment 3

In this embodiment, impurities included in an oxide semiconductor filmin a transistor and deterioration of the transistor characteristics aredescribed. In the description, the IGZO(111) is used for the oxidesemiconductor film and carbon is used as one of the impurities.

<1. Effect of Carbon in IGZO>

An electronic state was calculated on a model where a carbon atom wasintroduced into the IGZO(111).

For the calculation, a IGZO(111) crystal model (the number of atoms:112) shown in FIG. 29A was used.

Here, as the model where a carbon atom is included in the IGZO(111), asshown in FIG. 29A and Table 7, the following models were used: models ineach of which a carbon atom was put in the respective interstitial sites(1) to (6), a model where one indium atom was replaced with a carbonatom, a model where one gallium atom was replaced with a carbon atom, amodel where one zinc atom was replaced with a carbon atom, and a modelwhere one oxygen atom was replaced with a carbon atom.

TABLE 7 Arrangement Position Proximate metal atoms (1) between (Ga,Zn)Oand (Ga,Zn)O Ga4, Zn2 (2) Ga2, Zn4 (3) between (Ga,Zn)O and InO In3,Ga2, Zn1 (4) In3, Ga1, Zn2 (5) In1, Ga2, Zn1 (6) In1, Ga1, Zn2<1-1. Model where Carbon Atom was Put in Interstitial Site>

A stable configuration was examined by comparing the energy afterstructure optimization of the models where carbon atoms were put in therespective interstitial sites (1) to (6). The calculation conditions areshown in Table 8. Note that GGA was used for exchange-correlationfunction, and thus the band gap tended to be underestimated.

TABLE 8 Software VASP Model InGaZnO4 crystal (112 atoms) CalculationStructure optimization (fixed lattice constant) Functional GGA-PBEPseudopotential PAW method Cut-off energy 500 eV k-point 1 × 1 × 1(optimization), 3 × 3 × 4 (state density)

The results of the structure optimization calculation of the modelswhere carbon atoms were put in the respective interstitial sites (1) to(6) are shown in Table 9.

TABLE 9 Initial arrangement After optimization Energy (relative value)(1) interstitial site (Ga4, Zn2) (CO)o −618.511 eV (0.326 eV) (M1 = Ga,M2 = Ga, M3 = Zn, M4 = Zn) (2) interstitial site (Ga2, Zn4) interstitialsite (Ga2, Zn4) −615.091 eV (3.746 eV) (3) interstitial site (In3, Ga2,Zn1) (CO)o −618.640 eV (0.197 eV) (M1 = In, M2 = Ga, M3 = In, M4 = In)(4) interstitial site (In3, Ga1, Zn2) (CO)o −618.196 eV (0.641 eV) (M1 =In, M2 = Zn, M3 = In, M4 = In) (5) interstitial site (In1, Ga2, Zn1)bonded to In1, O3 −618.140 eV (0.697 eV) (6) interstitial site (In1,Ga1, Zn2) bonded to In1, O2 −618.837 eV (0.000 eV)

The interstitial sites were selected as the original position of acarbon atom. After the structure optimization was performed, a modelwhere a carbon atom was put in the interstitial site (1), (3), or (4)had a (CO)_(o) defect structure as illustrated in FIG. 29C. Note that(CO)_(o) means that one oxygen atom in the structure in FIG. 29B isreplaced with CO, as illustrated in the structure in FIG. 29C. In the(CO)_(o) defect structure, a carbon atom is bonded to an oxygen atom.The carbon atom is bonded to an atom M₁ and an atom M₂. The oxygen atomis bonded to an atom M₃ and an atom M₄. A model where a carbon atom wasput in the interstitial site (5) or (6) has a structure in which acarbon atom was bonded to atoms in the IGZO(111). When the energy wascompared, a carbon atom was more stable in the (CO)_(o) defect structureand in a structure where a carbon atom was bonded to the atoms in theIGZO(111) than in the interstitial site.

FIG. 30A shows a structure of the model that has the lowest energy andis the most stable (model where a carbon atom was put in theinterstitial site (6)) in the calculation. FIG. 30B shows the density ofstates. In FIG. 30B, when the Fermi level E_(F) is 0 eV in thehorizontal axis, the density of states of up-spin and down-spin areshown in the upper and lower sides of the Fermi level E_(F),respectively.

In the structure shown in FIG. 30A, a carbon atom is bonded to oneindium atom and two oxygen atoms. In a model where a silicon atombelonging to the same group as a carbon atom was put in the interstitialsite, the silicon atom was bonded only to an oxygen atom. The resultsindicate that the difference in bonding state between the silicon atomand the carbon atom may be attributed to differences of their ionicradiuses and electronegativity. In FIG. 30B, when the density of statesfrom the conduction band minimum to the Fermi level E_(F) is integrated,the density of states corresponds to two electrons. The Fermi levelE_(F) is positioned on the side closer to the vacuum level than theconduction band minimum is by two electrons; thus, it is presumed that,when the carbon atom is put in the interstitial site, two electrons arereleased from the carbon atom, so that the IGZO(111) becomes n-type.

<1-2. Model where Metal Element was Replaced with Carbon Atom>

FIGS. 31A and 31B show the optimal structure and density of states in amodel where one indium atom was replaced with a carbon atom. Note thatin the horizontal axis of FIG. 31B, the Fermi level E_(F) is 0 eV.

In the structure of FIG. 31A, a carbon atom is bonded to three oxygenatoms and positioned in a plane of a triangle having oxygen atoms as thevertexes. Although the sketch of the density of states illustrated inFIG. 31B is almost the same as that of the density of states in the caseof no defect, the Fermi level E_(F) is positioned on the side closer tothe vacuum level than the conduction band minimum is by one electron;thus, it is presumed that, when an indium atom is replaced with a carbonatom, one electron is released from the carbon atom, so that theIGZO(111) became n-type. This is probably because a trivalent indiumatom was replaced with a tetravalent carbon atom.

FIGS. 32A and 32B show the optimal structure and density of states in amodel where one gallium atom was replaced with a carbon atom. Note thatin the horizontal axis of FIG. 32B, the Fermi level E_(F) is 0 eV.

In the structure of FIG. 32A, a carbon atom is bonded to four oxygenatoms and positioned in almost the center of a tetrahedron having oxygenatoms as the vertexes. Although the sketch of the density of statesillustrated in FIG. 32B is almost the same as that of the density ofstates in the case of no defect, the Fermi level E_(F) is positioned onthe side closer to the vacuum level than the conduction band minimum isby one electron; thus, it is presumed that, when a gallium atom isreplaced with a carbon atom, one electron is released from the carbonatom, so that the IGZO(111) became n-type. This is probably because atrivalent gallium atom is replaced with a tetravalent carbon atom.

FIGS. 33A and 33B show the optimal structure and density of states in amodel where one zinc atom was replaced with a carbon atom. Note that inthe horizontal axis of FIG. 33B, the Fermi level E_(F) is 0 eV.

In the structure of FIG. 33A, a carbon atom is bonded to three oxygenatoms and positioned in a plane of a triangle having oxygen atoms as thevertexes. Although the sketch of the density of states illustrated inFIG. 33B is almost the same as that of the density of states in the caseof no defect, the Fermi level E_(F) is positioned on the side closer tothe vacuum level than the conduction band minimum is by two electrons;thus, it is presumed that, when a zinc atom is replaced with a carbonatom, two electrons are released from the carbon atom, so that theIGZO(111) became n-type. This is probably because a divalent zinc atomwas replaced with a tetravalent carbon atom.

<1-3. Model where Oxygen Atom was Replaced with Carbon Atom>

Next, whether an oxygen atom can be replaced with a carbon atom wasexamined. In the case where one oxygen atom is replaced with a carbonatom, there are four oxygen atom sites in consideration of a combinationof metals that are bonding partners of an oxygen atom, and substitutionmodels for the sites were formed and structure optimization calculationwas performed. As a result, a model where an oxygen atom bonded to twogallium atoms and one zinc atom was replaced with a carbon atom wasenergetically stable.

The IGZO(111) formed in an oxygen atmosphere contains sufficient oxygenatoms. Models (1) and (2) in Table 10 were examined to compare energiesneeded for a carbon atom to substitute for an oxygen atom in theIGZO(111) containing a lot of oxygen atoms. The numbers of atoms ofModel (1) and (2) were equalized; after that, the total energy of eachmodel was calculated.

TABLE 10 Model Existing mode (1) [InGaZnO₄] + [CO₂] (2) [InGaZnO₄:C_(O)] + 3/2[O₂]

In Model (1), a carbon atom was contained in the IGZO(111) as CO₂. InModel (2), an oxygen atom was replaced with a carbon atom in theIGZO(111).

The energy of Model (1) was calculated to be lower than that of Model(2) by approximately 10.8 eV, and thus Model (1) is more stable thanModel (2). This suggests that Model (1) is more likely to exist thanModel (2). That is, that presumably shows that an oxygen atom isunlikely to be replaced with a carbon atom and the state where an oxygenatom is replaced with a carbon atom is unstable.

In order to find out the stable configuration of a carbon atom, theIGZO(111) containing a lot of oxygen was assumed, and the total energyof models where the numbers of atoms are the same as each other werecalculated. Calculation results are shown in Table 11.

As shown in Table 11, it is presumed that, because the energy of agallium atom is low, a carbon atom in the IGZO(111) is likely tosubstitute for a gallium atom and is unlikely to substitute for anoxygen atom. Note that in Table 11, “IGZO:C_(atom)” means that the atomis replaced with a carbon atom in InGaZnO₄.

TABLE 11 Total energy Model Existing mode (relative value) (No defect)[InGaZnO₄] + [CO₂] −637.446 eV (0.000 eV) C is put in interstitial site[InGaZnO₄ + C] + [O₂] −628.695 eV (8.751 eV) In is replaced with C[InGaZnO₄: C_(In)] + 1/2[In₂O₃] + 1/4[O₂] −632.759 eV (4.687 eV) Ga isreplaced with C [InGaZnO₄: C_(Ga)] + 1/2[Ga₂O₃] + 1/4[O₂] −633.665 eV(3.781 eV) Zn is replaced with C [InGaZnO₄: C_(Zn)] + [ZnO] + 1/2[O₂]−632.801 eV (4.645 eV) O is replaced with C [InGaZnO₄: C_(O)] + 3/2[O₂]−626.620 eV (10.826 eV)

As a result, it is found that when a carbon atom is put in aninterstitial site or a carbon atom substitutes for a metal atom (In, Ga,or Zn), the IGZO(111) becomes n-type. Furthermore, it is presumed thatthe configuration becomes stable when a carbon atom in the IGZO(111)substitutes particularly for a gallium atom.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments andexamples.

Embodiment 4

In this embodiment, a semiconductor device and a manufacturing methodthereof, which are different from those in Embodiment 1, are describedwith reference to drawings. A transistor 50 of this embodiment is atop-gate transistor, which is different from the transistors inEmbodiment 1.

<1. Structure of Transistor>

FIGS. 34A to 34C are a top view and cross-sectional views of thetransistor 50. FIG. 34A is the top view of the transistor 50. FIG. 34Bis a cross-sectional view taken along dashed-dotted line A-B in FIG.34A. FIG. 34C is a cross-sectional view taken along dashed-dotted lineC-D in FIG. 34A. Note that in FIG. 34A, for simplicity, a substrate 51,a protective film 53, a gate insulating film 59, an insulating film 63,and the like are omitted.

The transistor 50 illustrated in FIGS. 34A to 34C includes an oxidesemiconductor film 55 over the protective film 53; a pair of electrodes57 and 58 in contact with the oxide semiconductor film 55; the gateinsulating film 59 in contact with the oxide semiconductor film 55 andthe pair of electrodes 57 and 58; and a gate electrode 61 overlappingwith the oxide semiconductor film 55 with the gate insulating film 59therebetween. The insulating film 63 may be provided over the protectivefilm 53, the pair of electrodes 57 and 58, the gate insulating film 59,and the gate electrode 61.

In this embodiment, a film in contact with the oxide semiconductor film55, typically, at least one of the protective film 53 and the gateinsulating film 59 is an oxide insulating film that contains nitrogenand has a small number of defects.

Typical examples of the oxide insulating film containing nitrogen andhaving a small number of defects include a silicon oxynitride film andan aluminum oxynitride film. Further, a “silicon oxynitride film” or an“aluminum oxynitride film” refers to a film that contains more oxygenthan nitrogen, and a “silicon nitride oxide film” or an “aluminumnitride oxide film” refers to a film that contains more nitrogen thanoxygen.

In an ESR spectrum at 100 K or lower of the oxide insulating film with asmall number of defects, a first signal that appears at a g-factor ofgreater than or equal to 2.037 and smaller than or equal to 2.039, asecond signal that appears at a g-factor of greater than or equal to2.001 and smaller than or equal to 2.003, and a third signal thatappears at a g-factor of greater than or equal to 1.964 and smaller thanor equal to 1.966 are observed. The sum of the spin densities of thefirst signal that appears at a g-factor of greater than or equal to2.037 and smaller than or equal to 2.039, the second signal that appearsat a g-factor of greater than or equal to 2.001 and smaller than orequal to 2.003, and the third signal that appears at a g-factor ofgreater than or equal to 1.964 and smaller than or equal to 1.966 islower than 1×10¹⁸ spins/cm³, typically higher than or equal to 1×10¹⁷spins/cm³ and lower than 1×10¹⁸ spins/cm³.

In the ESR spectrum at 100 K or lower, the first signal that appears ata g-factor of greater than or equal to 2.037 and smaller than or equalto 2.039, the second signal that appears at a g-factor of greater thanor equal to 2.001 and smaller than or equal to 2.003, and the thirdsignal that appears at a g-factor of greater than or equal to 1.964 andsmaller than or equal to 1.966 correspond to signals attributed tonitrogen oxide (NO_(x); x is greater than or equal to 0 and smaller thanor equal to 2, preferably greater than or equal to 1 and smaller than orequal to 2). Typical examples of nitrogen oxide include nitrogenmonoxide and nitrogen dioxide.

When at least one of the protective film 53 and the gate insulating film59 in contact with the oxide semiconductor film 55 contains a smallamount of nitrogen oxide as described above, the carrier trap at theinterface between the oxide semiconductor film 55 and the gateinsulating film 15 or the interface between the oxide semiconductor film55 and the protective film 21 can be inhibited. As a result, a change inthe threshold voltage of the transistor included in the semiconductordevice can be reduced, which leads to a reduced change in the electricalcharacteristics of the transistor.

At least one of the protective film 53 and the gate insulating film 59preferably has a nitrogen concentration measured by SIMS of lower thanor equal to 6×10²⁰ atoms/cm³. In that case, nitrogen oxide is unlikelyto be generated in at least one of the protective film 53 and the gateinsulating film 59, so that the carrier trap at the interface betweenthe oxide semiconductor film 55 and the gate insulating film 15 or theinterface between the oxide semiconductor film 55 and the protectivefilm 21 can be inhibited. Furthermore, a change in the threshold voltageof the transistor included in the semiconductor device can be reduced,which leads to a reduced change in the electrical characteristics of thetransistor.

The details of other components of the transistor 50 are describedbelow.

As the substrate 51, a substrate given as an example of the substrate 11of Embodiment 1 can be used as appropriate.

In the case where the gate insulating film 59 is formed of an oxideinsulating film containing nitrogen and having a small number ofdefects, the protective film 53 can be formed using an oxide insulatingfilm containing oxygen at a higher proportion than oxygen in thestoichiometric composition. The oxide insulating film containing oxygenat a higher proportion than oxygen in the stoichiometric composition candiffuse oxygen into an oxide semiconductor film by heat treatment. Astypical examples of the protective film 53, a silicon oxide film, asilicon oxynitride film, a silicon nitride oxide film, a gallium oxidefilm, a hafnium oxide film, an yttrium oxide film, an aluminum oxidefilm, an aluminum oxynitride film, and the like can be given.

The thickness of the protective film 53 is greater than or equal to 50nm, preferably greater than or equal to 200 nm and less than or equal to3000 nm, further preferably greater than or equal to 300 nm and lessthan or equal to 1000 nm. With use of the thick base the protective film53, the number of oxygen molecules released from the protective film 53can be increased, and the interface state at the interface between thebase the protective film 53 and an oxide semiconductor film formed latercan be reduced.

Here, “to release part of oxygen by heating” means that the amount ofreleased oxygen by conversion into oxygen atoms is greater than or equalto 1×10¹⁸ atoms/cm³, preferably greater than or equal to 3×10²⁰atoms/cm³ in TDS analysis. Note that the substrate temperature in theTDS analysis is preferably higher than or equal to 100° C. and lowerthan or equal to 700° C., or higher than or equal to 100° C. and lowerthan or equal to 500° C.

The oxide semiconductor film 55 can be formed in a manner similar tothat of the oxide semiconductor film 17 in Embodiment 1.

The pair of electrodes 57 and 58 can be formed in a manner similar tothat of the pair of electrodes 19 and 20 of Embodiment 1.

Note that although the pair of electrodes 57 and 58 are provided betweenthe oxide semiconductor film 55 and the gate insulating film 59 in thisembodiment, the pair of electrodes 57 and 58 may be provided between theprotective film 53 and the oxide semiconductor film 55.

In the case where the protective film 53 is formed using an oxideinsulating film containing nitrogen and having a small number ofdefects, the gate insulating film 59 can be formed to have asingle-layer structure or a stacked-layer structure using, for example,any of silicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, aluminum oxide, hafnium oxide, gallium oxide, Ga—Zn-based metaloxide, and the like. Note that an oxide insulating film is preferablyused for at least a region of the gate insulating film 59, which is incontact with the oxide semiconductor film 55, in order to improvecharacteristics of the interface with the oxide semiconductor film 55.

Further, it is possible to prevent outward diffusion of oxygen from theoxide semiconductor film 55 and entry of hydrogen, water, or the likeinto the oxide semiconductor film 55 from the outside by providing aninsulating film having a blocking effect against oxygen, hydrogen,water, and the like for the gate insulating film 59. As for theinsulating film having a blocking effect against oxygen, hydrogen,water, and the like, an aluminum oxide film, an aluminum oxynitridefilm, a gallium oxide film, a gallium oxynitride film, an yttrium oxidefilm, an yttrium oxynitride film, a hafnium oxide film, and a hafniumoxynitride film can be given as examples.

The gate insulating film 59 may be formed using a high-k material suchas hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current of the transistor can be reduced.

The thickness of the gate insulating film 59 is, for example, greaterthan or equal to 5 nm and less than or equal to 400 nm, preferablygreater than or equal to 10 nm and less than or equal to 300 nm, morepreferably greater than or equal to 15 nm and less than or equal to 100nm.

The gate electrode 61 can be formed in a manner similar to that of thegate electrode 13 of Embodiment 1.

The insulating film 63 is formed with a single-layer structure or astacked structure using one or more of silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, aluminum oxide,aluminum oxynitride, aluminum nitride oxide, aluminum nitride, and thelike to a thickness greater than or equal to 30 nm and less than orequal to 500 nm, preferably greater than or equal to 100 nm and lessthan or equal to 400 nm.

Like the protective film 53, the insulating film 63 may have astacked-layer structure including an oxynitride insulating filmcontaining oxygen at a higher proportion than oxygen in thestoichiometric composition and an insulating film having a blockingeffect against hydrogen, water, and the like. As the insulating filmhaving a blocking effect against oxygen, hydrogen, water, and the like,an aluminum oxide film, an aluminum oxynitride film, a gallium oxidefilm, a gallium oxynitride film, an yttrium oxide film, an yttriumoxynitride film, a hafnium oxide film, a hafnium oxynitride film, and asilicon nitride film can be given as examples. In the case where suchinsulating films are used, in heat treatment, oxygen is supplied to theoxide semiconductor film 55 through the gate insulating film 59 and/orthe protective film 53, which enables a reduction in the interface statebetween the oxide semiconductor film 55 and the gate insulating film 59and/or the interface state between the oxide semiconductor film 55 andthe protective film 53. Furthermore, the number of oxygen vacancies inthe oxide semiconductor film 55 can be reduced.

<2. Method for Manufacturing Transistor>

Next, a method for manufacturing the transistor illustrated in FIGS. 34Ato 34C is described with reference to FIGS. 35A to 35D. Across-sectional view in the channel length direction along dot-dashedline A-B in FIG. 34A and a cross-sectional view in the channel widthdirection along dot-dashed line C-D in FIG. 34A are used for describinga method for manufacturing the transistor 50.

The protective film 53 is formed over the substrate 51 as illustrated inFIG. 35A. Then, the oxide semiconductor film 55 is formed over theprotective film 53.

The protective film 53 is formed by a sputtering method, a CVD method,or the like.

In the case where an oxide insulating film containing nitrogen andhaving a small number of defects is formed as the protective film 53, asilicon oxynitride film can be formed by a CVD method as an example ofthe oxide insulating film containing nitrogen and having a small numberof defects. In this case, a deposition gas containing silicon and anoxidizing gas are preferably used as a source gas. Typical examples ofthe deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. Examples of the oxidizing gas includedinitrogen monoxide and nitrogen dioxide.

In the case where an oxide insulating film from which part of oxygen isreleased by heating is formed as the protective film 53, the oxideinsulating film is preferably formed by a sputtering method using theconditions where the amount of oxygen in a deposition gas is large. Asthe deposition gas, oxygen, a mixed gas of oxygen and a rare gas, or thelike can be used. Typically, the concentration of oxygen in thedeposition gas is preferably higher than or equal to 6% and lower thanor equal to 100%.

Furthermore in the case where an oxide insulating film from which partof oxygen is released by heating is formed as the protective film 53, anoxide insulating film is formed by a CVD method as the oxide insulatingfilm from which part of oxygen is released by heating, and then oxygenis introduced into the oxide insulating film, so that the amount ofoxygen released by heating can be increased. Oxygen can be added to theoxide insulating film by ion implantation, ion doping, plasma treatment,or the like. In this embodiment, the oxide semiconductor film is notprovided below the protective film 53; accordingly, even when oxygen isintroduced into the protective film 53, the oxide semiconductor film isnot damaged. Thus, oxygen can be introduced into the protective film 53in contact with the oxide semiconductor film without damage to the oxidesemiconductor film.

In the case where an oxide insulating film is formed by a CVD method asthe protective film 53, hydrogen or water derived from a source gas issometimes mixed in the oxide insulating film. Thus, after the oxideinsulating film is formed by a plasma CVD method, heat treatment ispreferably performed for dehydrogenation or dehydration.

The oxide semiconductor film 55 can be formed as appropriate by aformation method similar to that of the oxide semiconductor film 17described in Embodiment 1.

In order to improve the orientation of the crystal parts in the CAAC-OSfilm, planarity of the surface of the protective film 53 serving as abase insulating film of the oxide semiconductor film is preferablyimproved. Typically, the protective film 53 can be made to have anaverage surface roughness (Ra) of 1 nm or less, 0.3 nm or less, or 0.1nm or less.

As planarization treatment for improving planarity of the surface of theprotective film 53, one or more can be selected from chemical mechanicalpolishing (CMP) treatment, dry etching treatment, plasma treatment (whatis called reverse sputtering), and the like. The plasma treatment is theone in which minute unevenness of the surface is reduced by introducingan inert gas such as an argon gas into a vacuum chamber and applying anelectric field so that a surface to be processed serves as a cathode.

Next, as illustrated in FIG. 35B, the pair of electrodes 57 and 58 areformed. The pair of electrodes 57 and 58 can be formed as appropriate bya formation method similar to those of the pair of electrodes 19 and 20described in Embodiment 1. Alternatively, the pair of electrodes 57 and58 can be formed by a printing method or an inkjet method.

Next, as illustrated in FIG. 35C, the gate insulating film 59 and thegate electrode 61 are formed. An insulating film is formed by asputtering method, a CVD method, an evaporation method, or the like, anda conductive film is formed over the insulating film by a sputteringmethod, a CVD method, an evaporation method, or the like. Then, a maskis formed over the conductive film by a photolithography process. Afterthat, parts of insulating film and the conductive film are etched usingthe mask to form the gate insulating film 59 and the gate electrode 61.After that, the mask is removed.

A film to be the gate insulating film 59 is formed by a sputteringmethod, a CVD method, an evaporation method, or the like. A film to bethe gate electrode 61 is formed by a sputtering method, a CVD method, anevaporation method, or the like.

In the case where an oxide insulating film containing nitrogen andhaving a small number of defects is formed as the film to be the gateinsulating film 59, the film can be formed using conditions similar tothose of the protective film 53 as appropriate.

Next, as illustrated in FIG. 35D, the insulating film 63 is formed overthe substrate 51, the pair of electrodes 57 and 58, the gate insulatingfilm 59, and the gate electrode 61. The base insulating film 63 can beformed as appropriate by a sputtering method, a CVD method, a printingmethod, a coating method, or the like.

Next, in a manner similar to that in Embodiment 1, heat treatment may beperformed. The heat treatment is performed typically at a temperaturehigher than or equal to 150° C. and lower than the strain point of thesubstrate, preferably higher than or equal to 250° C. and lower than orequal to 450° C., more preferably higher than or equal to 300° C. andlower than or equal to 450° C.

Through the above steps, a transistor in which a change in thresholdvoltage is reduced can be manufactured. Further, a transistor in which achange in electrical characteristics is reduced can be manufactured.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments andexamples.

Modification Example 1

Modification examples of the transistor 50 described in Embodiment 4 aredescribed with reference to FIGS. 36A and 36B. In each of thetransistors described in this modification example, a gate insulatingfilm or a protective film has a stacked-layer structure.

In a transistor 50 a illustrated in FIG. 36A, the protective film 53have a multi-layer structure. Specifically, in the protective film 53,an oxide insulating film 65 and an oxide insulating film 67 are stacked.The oxide insulating film 65 contains oxygen at a higher proportion thanoxygen in the stoichiometric composition. The oxide insulating film 67in contact with the oxide semiconductor film 55 contains nitrogen, has asmall number of defects, and can be used as at least one of theprotective film 53 and the gate insulating film 59 of the transistor 50.

The oxide insulating film 65 containing oxygen at a higher proportionthan oxygen in the stoichiometric composition has a thickness of greaterthan or equal to 50 nm, preferably greater than or equal to 200 nm andless than or equal to 3000 nm, more preferably greater than or equal to300 nm and less than or equal to 1000 nm. When the oxide insulating film65 containing oxygen at a higher proportion than oxygen in thestoichiometric composition is formed thick, the number of releasedoxygen molecules in the oxide insulating film 65 containing oxygen at ahigher proportion than oxygen in the stoichiometric composition can beincreased, and the interface state at the interface between the oxideinsulating film 67 and the oxide semiconductor film 55 can be lowered.

For forming the oxide insulating film 65 containing oxygen at a higherproportion than oxygen in the stoichiometric composition, an oxideinsulating film from which part of oxygen is released by heating andwhich can be used as the protective film 53 can be used as appropriate.

Furthermore, the oxide insulating film 67 can be formed in the formationmanner of the oxide insulating film containing nitrogen and having asmall number of defects which can be used as the protective film 53 andthe gate insulating film 59 in the transistor 50.

The oxide insulating film 65 containing oxygen at a higher proportionthan oxygen in the stoichiometric composition and the oxide insulatingfilm 67 are formed, and the oxide semiconductor film 55 is formed overthe oxide insulating film 67. After that, heat treatment may beperformed. By the heat treatment, part of oxygen contained in the oxideinsulating film 65 containing oxygen at a higher proportion than oxygenin the stoichiometric composition can be diffused in the vicinity of theinterface between the oxide insulating film 67 and the oxidesemiconductor film 55. As a result, the interface state in the vicinityof the interface between the oxide insulating film 67 and the oxidesemiconductor film 55 can be lowered, so that a change in thresholdvoltage can be reduced.

The temperature of the heat treatment is typically higher than or equalto 150° C. and lower than the strain point of the substrate, preferablyhigher than or equal to 250° C. and lower than or equal to 450° C., morepreferably higher than or equal to 300° C. and lower than or equal to450° C.

The heat treatment is performed under an inert gas atmosphere containingnitrogen or a rare gas such as helium, neon, argon, xenon, or krypton.Further, the heat treatment may be performed under an inert gasatmosphere first, and then under an oxygen atmosphere. It is preferablethat the above inert gas atmosphere and the above oxygen atmosphere donot contain hydrogen, water, and the like. The treatment time is 3minutes to 24 hours.

In a transistor 50 b illustrated in FIG. 36B, the gate insulating film59 has a stacked structure of an oxide insulating film 69 and a nitrideinsulating film 71 in this order, and the oxide insulating film 69 incontact with the oxide semiconductor film 55 is an oxide insulating filmcontaining nitrogen and having a small number of defects.

As the nitride insulating film 71, a film similar to the nitrideinsulating film 29 described in Modification example 1 in Embodiment 1is preferably used. Thus, the physical thickness of the gate insulatingfilm 59 can be increased. This makes it possible to reduce a decrease inwithstand voltage of the transistor 50 b and furthermore increase thewithstand voltage, thereby reducing electrostatic discharge damage to asemiconductor device.

Modification Example 2

A modification example of the transistor 50 described in Embodiment 4 isdescribed with reference to FIGS. 37A to 37C. In this modificationexample, a transistor in which an oxide semiconductor film is providedbetween a gate insulating film and a pair of electrodes is described.

FIGS. 37A to 37C are a top view and cross-sectional views of atransistor 50 c included in a semiconductor device of one embodiment ofthe present invention. FIG. 37A is a top view, FIG. 37B is a schematiccross-sectional view taken along dot-dashed line A-B in FIG. 37A, andFIG. 37C is a schematic cross-sectional view taken along dot-dashed lineC-D in FIG. 37A.

The transistor 50 c illustrated in FIGS. 37B and 37C includes an oxidesemiconductor film 73 over the protective film 53; the oxidesemiconductor film 55 over the oxide semiconductor film 73; the pair ofelectrodes 57 and 58 in contact with the oxide semiconductor film 55 andthe oxide semiconductor film 73; an oxide semiconductor film 75 incontact with the oxide semiconductor film 55 and the pair of electrodes57 and 58; the gate insulating film 59 over the oxide semiconductor film75; and the gate electrode 61 overlapping with the oxide semiconductorfilm 55 with the gate insulating film 59 therebetween. The insulatingfilm 63 may be provided over the protective film 53, the pair ofelectrodes 57 and 58, the oxide semiconductor film 75, the gateinsulating film 59, and the gate electrode 61.

In the transistor 50 c, the protective film 53 has a projecting portion,and the stacked oxide semiconductor films 73 and 55 are provided overthe projecting portion of the protective film 53.

As illustrated in FIG. 37B, the oxide semiconductor film 75 is incontact with the top surface of the oxide semiconductor film 55 and thetop and side surfaces of the pair of electrodes 57 and 58. Asillustrated in FIG. 37C, the oxide semiconductor film 75 is in contactwith a side surface of the projecting portion of the protective film 53,a side surface of the oxide semiconductor film 73, and the top and sidesurfaces of the oxide semiconductor film 55.

As illustrated in FIG. 37C, in the channel width direction of thetransistor 50 c, the gate electrode 61 faces the top and side surfacesof the oxide semiconductor film 55 with the oxide semiconductor film 75and the gate insulating film 59 therebetween.

The gate electrode 61 electrically surrounds the oxide semiconductorfilm 55. With this structure, on-state current of the transistor 50 ccan be increased. Such a transistor structure is referred to as asurrounded channel (s-channel) structure. Note that in the s-channelstructure, current flows in the whole (bulk) of the oxide semiconductorfilm 55. Since current flows in an inner part of the oxide semiconductorfilm 55, the current is hardly affected by interface scattering, andhigh on-state current can be obtained. In addition, by making the oxidesemiconductor film 55 thick, on-state current can be increased.

In fabricating a transistor with a small channel length and a smallchannel width, when a pair of electrodes, an oxide semiconductor film,or the like is processed while a resist mask is reduced in size, thepair of electrodes, the oxide semiconductor film, or the like has around end portion (curved surface) in some cases. With this structure,the coverage with the oxide semiconductor film 75 and the gateinsulating film 59, which are to be formed over the oxide semiconductorfilm 55, can be improved. In addition, electric field concentrationwhich might occur at the edges of the pair of electrodes 57 and 58 canbe relaxed, which can suppress deterioration of the transistor.

In addition, by miniaturizing the transistor, higher integration andhigher density can be achieved. For example, the channel length of thetransistor is set to 100 nm or less, preferably 40 nm or less, morepreferably 30 nm or less, still more preferably 20 nm or less, and thechannel width of the transistor is set to 100 nm or less, preferably 40nm or less, more preferably 30 nm or less, still more preferably 20 nmor less. The transistor of one embodiment of the present invention withthe s-channel structure can increase on-state current even in the casewhere the channel width thereof is shortened as described above.

For the oxide semiconductor film 73, the material of the oxidesemiconductor film 46 described in Modification example 4 in Embodiment1 can be used as appropriate. Before a film to be the oxidesemiconductor film 55 is formed in FIG. 35A, a film to be the oxidesemiconductor film 73 is formed. Then, a film to be the oxidesemiconductor film 73 and a film to be the oxide semiconductor film 55are processed, whereby the oxide semiconductor film 73 and the oxidesemiconductor film 55 can be obtained.

For the oxide semiconductor film 75, the material of the oxidesemiconductor film 47 described in Modification example 4 in Embodiment1 can be used as appropriate. Before a film to be the gate insulatingfilm 59 is formed in FIG. 35C, a film to be the oxide semiconductor film75 is formed. Then, a film to be the gate insulating film 59 and a filmto be the gate electrode 61 are formed. After that, the films areprocessed at the same time, whereby the oxide semiconductor film 75, thegate insulating film 59, and the gate electrode 61 can be obtained.

High integration of a semiconductor device requires miniaturization of atransistor. However, it is known that miniaturization of a transistorcauses deterioration in electrical characteristics of the transistor. Adecrease in channel width causes a reduction in on-state current.

However, in the transistor of one embodiment of the present invention,as described above, the oxide semiconductor film 75 is formed to coverthe channel formation region of the oxide semiconductor film 55, and thechannel formation region and the gate insulating film 59 are not incontact with each other. Therefore, scattering of carries formed at theinterface between the oxide semiconductor film 55 and the gateinsulating film 59 can be suppressed, whereby on-state current of thetransistor can be increased.

In the case where an oxide semiconductor film is made intrinsic orsubstantially intrinsic, decrease in the number of carriers contained inthe oxide semiconductor film may reduce the field-effect mobility.However, in the transistor of one embodiment of the present invention, agate electric field is applied to the oxide semiconductor film 55 notonly in the vertical direction but also from the side surfaces. That is,the gate electric field is applied to the whole of the oxidesemiconductor film 55, whereby current flows in the bulk of the oxidesemiconductor films. It is thus possible to improve the field-effectmobility of the transistor while a change in electrical characteristicsis reduced by highly purified intrinsic properties.

In the transistor of one embodiment of the present invention, the oxidesemiconductor film 55 is formed over the oxide semiconductor film 73, sothat an interface state is less likely to be formed. In addition,impurities do not enter the oxide semiconductor film 55 from above andbelow because the oxide semiconductor film 55 are provided between theoxide semiconductor films 73 and 75. Thus, the oxide semiconductor film55 is surrounded by the oxide semiconductor film 73 and the oxidesemiconductor film 75 (also electrically surrounded by the gateelectrode 61), so that stabilization of the threshold voltage inaddition to the above-described improvement of on-state current of thetransistor is possible. As a result, current flowing between the sourceand the drain when the voltage of the gate electrode is 0 V can bereduced, which leads to lower power consumption. Further, the thresholdvoltage of the transistor becomes stable; thus, long-term reliability ofthe semiconductor device can be improved.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments andexamples.

Embodiment 5

In this embodiment, one embodiment that can be applied to the oxidesemiconductor film in any of the transistors included in thesemiconductor device described in the above embodiment is described.

The oxide semiconductor film may include one or more of the following:an oxide semiconductor having a single-crystal structure (hereinafterreferred to as a single-crystal oxide semiconductor); an oxidesemiconductor having a polycrystalline structure (hereinafter referredto as a polycrystalline oxide semiconductor); an oxide semiconductorhaving a microcrystalline structure (hereinafter referred to as amicrocrystalline oxide semiconductor), and an oxide semiconductor havingan amorphous structure (hereinafter referred to as an amorphous oxidesemiconductor). Further, the oxide semiconductor film may be formedusing a CAAC-OS film. Furthermore, the oxide semiconductor film mayinclude an amorphous oxide semiconductor and an oxide semiconductorhaving a crystal grain. Described below are the CAAC-OS and themicrocrystalline oxide semiconductor.

<1. CAAC-OS>

The CAAC-OS film is one of oxide semiconductor films having a pluralityof crystal parts. The crystal parts included in the CAAC-OS film eachhave c-axis alignment. In a plan TEM image, the area of the crystalparts included in the CAAC oxide film is greater than or equal to 2500nm², preferably greater than or equal to 5 μm², more preferably greaterthan or equal to 1000 μm². Further, in a cross-sectional TEM image, whenthe proportion of the crystal parts is greater than or equal to 50%,preferably greater than or equal to 80%, more preferably greater than orequal to 95% of the CAAC-OS film, the CAAC-OS film is a thin film havingphysical properties similar to those of a single crystal.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film. In this specification, aterm “parallel” indicates that the angle formed between two straightlines is greater than or equal to −10° and less than or equal to 10°,and accordingly also includes the case where the angle is greater thanor equal to −5° and less than or equal to 5°. In addition, a term“perpendicular” indicates that the angle formed between two straightlines is greater than or equal to 80° and less than or equal to 100°,and accordingly includes the case where the angle is greater than orequal to 85° and less than or equal to 95°.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

Note that in an electron diffraction pattern of the CAAC-OS film, spots(luminescent spots) having alignment are shown.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. When the CAAC-OS film is analyzed by anout-of-plane method, a peak appears frequently when the diffractionangle (2θ) is around 31°. This peak is derived from the (00x) plane (xis an integer) of the InGaZn oxide crystal, which indicates thatcrystals in the CAAC-OS film have c-axis alignment, and that the c-axesare aligned in a direction substantially perpendicular to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an In-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZn oxidecrystal. Here, analysis (φ scan) is performed under the conditions wherethe sample is rotated around a normal vector of a sample surface as anaxis (φ axis) with 2θ fixed at around 56°. In the case where the sampleis a single-crystal oxide semiconductor film of InGaZn oxide, six peaksappear. The six peaks are derived from crystal planes equivalent to the(110) plane. On the other hand, in the case of a CAAC-OS film, a peak isnot clearly observed even when 0 scan is performed with 2θ fixed ataround 56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned witha direction parallel to a normal vector of a formation surface or anormal vector of a top surface. Thus, for example, in the case where ashape of the CAAC-OS film is changed by etching or the like, the c-axismight not be necessarily parallel to a normal vector of a formationsurface or a normal vector of a top surface of the CAAC-OS film.

Further, the degree of crystallinity in the CAAC-OS film is notnecessarily uniform. For example, in the case where crystal growthleading to the CAAC-OS film occurs from the vicinity of the top surfaceof the film, the degree of the crystallinity in the vicinity of the topsurface is higher than that in the vicinity of the formation surface insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystallinity in a region to which the impurity is added is changed, andthe degree of crystallinity in the CAAC-OS film varies depending onregions.

Note that when the CAAC-OS film is analyzed by an out-of-plane method, apeak of 2θ may also be observed at around 36°, in addition to the peakof 2θ at around 31°. The peak of 2θ at around 36° indicates that acrystal part having no c-axis alignment is included in part of theCAAC-OS film. It is preferable that in the CAAC-OS film, a peak of 20appear at around 31° and a peak of 20 not appear at around 36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is trapped therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Thus, a transistorincluding the oxide semiconductor film rarely has negative thresholdvoltage (is rarely normally on). The highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has alow density of defect states, and thus has few carrier traps.Accordingly, the transistor including the oxide semiconductor film haslittle variation in electrical characteristics and high reliability.Charges trapped by the carrier traps in the oxide semiconductor filmtake a long time to be released, and might behave like fixed charges.Thus, the transistor that includes the oxide semiconductor film havinghigh impurity concentration and a high density of defect states hasunstable electrical characteristics in some cases.

With the use of the CAAC-OS film in a transistor, variation inelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

<2. Microcrystalline Oxide Semiconductor>

In an image obtained with the TEM, crystal parts cannot be found clearlyin the microcrystalline oxide semiconductor in some cases. In mostcases, the size of a crystal part in a microcrystalline oxidesemiconductor film is greater than or equal to 1 nm and less than orequal to 100 nm, or greater than or equal to 1 nm and less than or equalto 10 nm. A microcrystal with a size greater than or equal to 1 nm andless than or equal to 10 nm, or a size greater than or equal to 1 nm andless than or equal to 3 nm is specifically referred to as nanocrystal(nc). An oxide semiconductor film including nanocrystal is referred toas an nc-OS (nanocrystalline oxide semiconductor) film. In an imageobtained with TEM, a crystal grain boundary cannot be found clearly inthe nc-OS film in some cases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. Further, there is noregularity of crystal orientation between different crystal parts in thenc-OS film; thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is subjected to structural analysis byan out-of-plane method with an XRD apparatus using an X-ray having adiameter larger than the diameter of a crystal part, a peak that shows acrystal plane does not appear. Further, a halo pattern is shown in aselected-area electron diffraction pattern of the nc-OS film obtained byusing an electron beam having a diameter larger than the diameter of acrystal part (e.g., larger than or equal to 50 nm). Meanwhile, spots areshown in a nanobeam electron diffraction pattern of the nc-OS filmobtained by using an electron beam having a probe diameter (e.g., largerthan or equal to 1 nm and smaller than or equal to 30 nm) close to, orsmaller than or equal to the diameter of a crystal part. Further, in ananobeam electron diffraction pattern of the nc-OS film, regions withhigh luminance in a circular (ring) pattern are shown in some cases.Also in a nanobeam electron diffraction pattern of the nc-OS film, aplurality of spots are shown in a ring-like region in some cases.

since the nc-OS film is an oxide semiconductor film having moreregularity than the amorphous oxide semiconductor film, the nc-OS filmhas a lower density of defect states than the amorphous oxidesemiconductor film. However, there is no regularity of crystalorientation between different crystal parts in the nc-OS film; hence,the nc-OS film has a higher density of defect states than the CAAC-OSfilm.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments andexamples.

Embodiment 6

In this embodiment, a structural example of a display panel of oneembodiment of the present invention is described.

<Structural Example>

FIG. 38A is a top view of the display panel of one embodiment of thepresent invention. FIG. 38B is a circuit diagram illustrating a pixelcircuit that can be used in the case where a liquid crystal element isused in a pixel in the display panel of one embodiment of the presentinvention. FIG. 38C is a circuit diagram illustrating a pixel circuitthat can be used in the case where an organic EL element is used in apixel in the display panel of one embodiment of the present invention.

The transistor in the pixel portion can be formed in accordance with theabove embodiments. Further, the transistor can easily be an n-channeltransistor, and thus, part of a driver circuit that can be formed usingan n-channel transistor in the driver circuit is formed over the samesubstrate as the transistor of the pixel portion. With the use of any ofthe transistors described in the above embodiments for the pixel portionor the driver circuit in this manner, a highly reliable display devicecan be provided.

FIG. 38A illustrates an example of a block diagram of an active matrixdisplay device. A pixel portion 901, a first scan line driver circuit902, a second scan line driver circuit 903, and a signal line drivercircuit 904 are provided over a substrate 900 in the display device. Inthe pixel portion 901, a plurality of signal lines extended from thesignal line driver circuit 904 are arranged, and a plurality of scanlines extended from the first scan line driver circuit 902 and thesecond scan line driver circuit 903 are arranged. Pixels each includinga display element are provided in matrix in respective regions in eachof which the scan line and the signal line intersect with each other.The substrate 900 of the display device is connected to a timing controlcircuit (also referred to as controller or control IC) through aconnection portion such as a flexible printed circuit (FPC).

In FIG. 38A, the first scan line driver circuit 902, the second scanline driver circuit 903, and the signal line driver circuit 904 areformed over the same substrate 900 as the pixel portion 901.Accordingly, the number of components such as a driver circuit, whichare provided outside, is reduced, so that a reduction in cost can beachieved. Further, if the driver circuit is provided outside thesubstrate 900, wirings would need to be extended and the number ofwiring connections would increase. However, by providing the drivercircuit over the substrate 900, the number of wiring connections can bereduced and the reliability or yield can be improved.

<Liquid Crystal Panel>

FIG. 38B illustrates an example of a circuit configuration of the pixel.Here, a pixel circuit that can be used in a pixel of a VA liquid crystaldisplay panel is illustrated.

This pixel circuit can be used in a structure in which one pixelincludes a plurality of pixel electrodes. The pixel electrodes areconnected to different transistors, and the transistors can be drivenwith different gate signals. Accordingly, signals applied to individualpixel electrodes in a multi-domain pixel can be controlledindependently.

A gate wiring 912 of a transistor 916 and a gate wiring 913 of atransistor 917 are separated so that different gate signals can be giventhereto. In contrast, a source or drain electrode 914 serving as a dataline is used in common for the transistors 916 and 917. Any of thetransistors described in the above embodiments can be used asappropriate as each of the transistors 916 and 917. In this way, ahighly reliable liquid crystal display panel can be provided.

The shapes of a first pixel electrode electrically connected to thetransistor 916 and a second pixel electrode electrically connected tothe transistor 917 are described. The first pixel electrode and thesecond pixel electrode are separated by a slit. The first pixelelectrode has a V shape and the second pixel electrode is provided so asto surround the first pixel electrode.

A gate electrode of the transistor 916 is connected to the gate wiring912, and a gate electrode of the transistor 917 is connected to the gatewiring 913. When different gate signals are supplied to the gate wiring912 and the gate wiring 913, operation timings of the transistor 916 andthe transistor 917 can be varied. As a result, alignment of liquidcrystals can be controlled.

Further, a storage capacitor may be formed using a capacitor wiring 910,a gate insulating film serving as a dielectric, and a capacitorelectrode electrically connected to the first pixel electrode or thesecond pixel electrode.

The multi-domain pixel includes a first liquid crystal element 918 and asecond liquid crystal element 919. The first liquid crystal element 918includes the first pixel electrode, a counter electrode, and a liquidcrystal layer therebetween. The second liquid crystal element 919includes the second pixel electrode, a counter electrode, and a liquidcrystal layer therebetween.

Note that a pixel circuit of the present invention is not limited tothat shown in FIG. 38B. For example, a switch, a resistor, a capacitor,a transistor, a sensor, a logic circuit, or the like may be added to thepixel illustrated in FIG. 38B.

<Organic EL Panel>

FIG. 38C illustrates another example of a circuit configuration of thepixel. Here, a pixel structure of a display panel using an organic ELelement is illustrated.

In an organic EL element, by application of voltage to a light-emittingelement, electrons are injected from one of a pair of electrodes andholes are injected from the other of the pair of electrodes, into alayer containing a light-emitting organic compound; thus, current flows.The electrons and holes are recombined, and thus, the light-emittingorganic compound is excited. The light-emitting organic compound returnsto a ground state from the excited state, thereby emitting light. Basedon such a mechanism, such a light-emitting element is referred to as acurrent-excitation type light-emitting element.

FIG. 38C illustrates an example of a pixel circuit that can be used.Here, an example in which an n-channel transistor is used in the pixelis shown. Further, digital time grayscale driving can be employed forthe pixel circuit.

The configuration of the pixel circuit that can be used and operation ofa pixel employing digital time grayscale driving are described.

A pixel 920 includes a switching transistor 921, a driving transistor922, a light-emitting element 924, and a capacitor 923. A gate electrodeof the switching transistor 921 is connected to a scan line 926. A firstelectrode (one of a source electrode and a drain electrode) of theswitching transistor 921 is connected to a signal line 925. A secondelectrode (the other of the source electrode and the drain electrode) ofthe switching transistor 921 is connected to a gate electrode of thedriving transistor 922. The gate electrode of the driving transistor 922is connected to a power supply line 927 through the capacitor 923, afirst electrode of the driving transistor 922 is connected to the powersupply line 927, and a second electrode of the driving transistor 922 isconnected to a first electrode (pixel electrode) of the light-emittingelement 924. A second electrode of the light-emitting element 924corresponds to a common electrode 928. The common electrode 928 iselectrically connected to a common potential line formed over the samesubstrate as the common electrode 928.

As the switching transistor 921 and the driving transistor 922, any ofthe transistors described in the above embodiments can be used asappropriate. In this way, a highly reliable organic EL display panel canbe provided.

The potential of the second electrode (the common electrode 928) of thelight-emitting element 924 is set to be a low power supply potential.Note that the low power supply potential is lower than a high powersupply potential supplied to the power supply line 927. For example, thelow power supply potential can be GND, 0 V, or the like. The high powersupply potential and the low power supply potential are set to be higherthan or equal to the forward threshold voltage of the light-emittingelement 924, and the difference between the potentials is applied to thelight-emitting element 924, whereby current is supplied to thelight-emitting element 924, leading to light emission. The forwardvoltage of the light-emitting element 924 refers to a voltage at which adesired luminance is obtained, and at least includes a forward thresholdvoltage.

Note that gate capacitance of the driving transistor 922 may be used asa substitute for the capacitor 923, so that the capacitor 923 can beomitted. The gate capacitance of the driving transistor 922 may beformed between the semiconductor film and the gate electrode.

Next, a signal input to the driving transistor 922 is described. For avoltage-input voltage driving method, a video signal for turning on oroff the driving transistor 922 without fail is input to the drivingtransistor 922. In order for the driving transistor 922 to operate in asubthreshold region, voltage higher than the voltage of the power supplyline 927 is applied to the gate electrode of the driving transistor 922.Voltage higher than or equal to voltage that is the sum of power supplyline voltage and the threshold voltage V_(th) of the driving transistor922 is applied to the signal line 925.

In the case where analog grayscale driving is performed, voltage higherthan or equal to voltage that is the sum of the forward voltage of thelight-emitting element 924 and the threshold voltage V_(th) of thedriving transistor 922 is applied to the gate electrode of the drivingtransistor 922. A video signal by which the driving transistor 922 isoperated in a saturation region is input, so that current is supplied tothe light-emitting element 924. In order for the driving transistor 922to operate in a saturation region, the potential of the power supplyline 927 is set higher than the gate potential of the driving transistor922. When an analog video signal is used, current corresponding to thevideo signal can be supplied to the light-emitting element 924 andanalog grayscale driving can be performed.

Note that the configuration of the pixel circuit is not limited to thatshown in FIG. 38C. For example, a switch, a resistor, a capacitor, asensor, a transistor, a logic circuit, or the like may be added to thepixel circuit illustrated in FIG. 38C.

In the case where the transistor described in the above embodiments isused for the circuit shown in FIGS. 38A to 38C, the source electrode(the first electrode) is electrically connected to the low potentialside and the drain electrode (the second electrode) is electricallyconnected to the high potential side. Further, the potential of thefirst gate electrode (and the third gate electrode) may be controlled bya control circuit or the like, and a potential lower than the potentialapplied to the source electrode may be input to the second gateelectrode through a wiring that is not illustrated.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

Embodiment 7

In this embodiment, a display module and electronic devices that can beformed using a semiconductor device of one embodiment of the presentinvention are described.

In a display module 8000 illustrated in FIG. 39, a touch panel 8004connected to an FPC 8003, a display panel 8006 connected to an FPC 8005,a backlight unit 8007, a frame 8009, a printed board 8010, and a battery8011 are provided between an upper cover 8001 and a lower cover 8002.Note that the backlight unit 8007, the battery 8011, the touch panel8004, and the like are not provided in some cases.

The semiconductor device of one embodiment of the present invention canbe used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and can be formed to overlap with the display panel 8006. Acounter substrate (sealing substrate) of the display panel 8006 can havea touch panel function. A photosensor may be provided in each pixel ofthe display panel 8006 to form an optical touch panel. An electrode fora touch sensor may be provided in each pixel of the display panel 8006so that a capacitive touch panel is obtained.

The backlight unit 8007 includes a light source 8008. The light source8008 may be provided at an end portion of the backlight unit 8007 and alight diffusing plate may be used.

The frame 8009 protects the display panel 8006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 may function asa radiator plate.

The printed board 8010 is provided with a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or a power source using thebattery 8011 provided separately may be used. The battery 8011 can beomitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

FIGS. 40A to 40D are external views of electronic devices each includingthe semiconductor device of one embodiment of the present invention.

Examples of electronic devices are a television set (also referred to asa television or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game machine, aportable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.

FIG. 40A illustrates a portable information terminal including a mainbody 1001, a housing 1002, display portions 1003 a and 1003 b, and thelike. The display portion 1003 b is a touch panel. By touching akeyboard button 1004 displayed on the display portion 1003 b, a screencan be operated, and text can be input. It is needless to say that thedisplay portion 1003 a may be a touch panel. A liquid crystal panel oran organic light-emitting panel is manufactured by using any of thetransistors described in the above embodiments as a switching elementand used in the display portion 1003 a or 1003 b, whereby a highlyreliable portable information terminal can be provided.

The portable information terminal illustrated in FIG. 40A has a functionof displaying various kinds of data (e.g., a still image, a movingimage, and a text image) on the display portion, a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a function of operating or editing the data displayed on thedisplay portion, a function of controlling processing by various kindsof software (programs), and the like. Furthermore, an externalconnection terminal (an earphone terminal, a USB terminal, or the like),a recording medium insertion portion, and the like may be provided onthe back surface or the side surface of the housing.

The portable information terminal illustrated in FIG. 40A may transmitand receive data wirelessly. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an e-bookserver.

FIG. 40B illustrates a portable music player including, in a main body1021, a display portion 1023, a fixing portion 1022 with which theportable music player can be worn on the ear, a speaker, an operationbutton 1024, an external memory slot 1025, and the like. A liquidcrystal panel or an organic light-emitting panel is fabricated using anyof the transistors described in the above embodiments as a switchingelement, and used in the display portion 1023, whereby a highly reliableportable music player can be provided.

Furthermore, when the portable music player illustrated in FIG. 40B hasan antenna, a microphone function, or a wireless communication functionand is used with a mobile phone, a user can talk on the phone wirelesslyin a hands-free way while driving a car or the like.

FIG. 40C illustrates a mobile phone that includes two housings, ahousing 1030 and a housing 1031. The housing 1031 includes a displaypanel 1032, a speaker 1033, a microphone 1034, a pointing device 1036, acamera lens 1037, an external connection terminal 1038, and the like.The housing 1030 is provided with a solar cell 1040 for charging themobile phone, an external memory slot 1041, and the like. In addition,an antenna is incorporated in the housing 1031. Any of the transistorsdescribed in the above embodiments is used in the display panel 1032,whereby a highly reliable mobile phone can be provided.

Further, the display panel 1032 includes a touch panel. A plurality ofoperation keys 1035 that are displayed as images are indicated by dottedlines in FIG. 40C. Note that a boosting circuit by which a voltageoutput from the solar cell 1040 is increased to be sufficiently high foreach circuit is also included.

In the display panel 1032, the direction of display is changed asappropriate depending on the application mode. Further, the mobile phoneis provided with the camera lens 1037 on the same surface as the displaypanel 1032, and thus it can be used as a video phone. The speaker 1033and the microphone 1034 can be used for videophone calls, recording, andplaying sound, etc. as well as voice calls. Moreover, the housings 1030and 1031 in a state where they are opened as illustrated in FIG. 40C canshift, by sliding, to a state where one is lapped over the other.Therefore, the size of the mobile phone can be reduced, which makes themobile phone suitable for being carried around.

The external connection terminal 1038 can be connected to an AC adaptorand a variety of cables such as a USB cable, whereby charging and datacommunication with a personal computer or the like are possible.Further, by inserting a recording medium into the external memory slot1041, a larger amount of data can be stored and moved.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be provided.

FIG. 40D illustrates an example of a television set. In a television set1050, a display portion 1053 is incorporated in a housing 1051. Imagescan be displayed on the display portion 1053. Moreover, a CPU isincorporated in a stand 1055 for supporting the housing 1051. Any of thetransistors described in the above embodiments is used in the displayportion 1053 and the CPU, whereby the television set 1050 can have highreliability.

The television set 1050 can be operated with an operation switch of thehousing 1051 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

Note that the television set 1050 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television set isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

Further, the television set 1050 is provided with an external connectionterminal 1054, a storage medium recording and reproducing portion 1052,and an external memory slot. The external connection terminal 1054 canbe connected to various types of cables such as a USB cable, and datacommunication with a personal computer or the like is possible. A diskstorage medium is inserted into the storage medium recording andreproducing portion 1052, and reading data stored in the storage mediumand writing data to the storage medium can be performed. In addition, animage, a video, or the like stored as data in an external memory 1056inserted into the external memory slot can be displayed on the displayportion 1053.

Further, in the case where the off-state leakage current of thetransistor described in the above embodiments is extremely small, whenthe transistor is used in the external memory 1056 or the CPU, thetelevision set 1050 can have high reliability and sufficiently reducedpower consumption.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

Example 1

In this example, results of evaluating an oxide insulating film that canbe used for the transistor of one embodiment of the present inventionare described. More specifically, results of evaluating, by TDS, theamounts of nitrogen monoxide, dinitrogen monoxide, nitrogen dioxide, andnitrogen released by heating are described.

<Fabrication Method of Samples>

In this example, Sample A1 that is one embodiment of the presentinvention and Samples A2 and A3 for comparison were fabricated.

<Sample A1>

Sample A1 was fabricated by forming an oxide insulating film over asilicon wafer by a plasma CVD method under formation conditions that canbe used for at least one of the gate insulating film 15 and theprotective film 21 described in Embodiment 1 (see FIG. 1).

Here, as the oxide insulating film, a 400-nm-thick silicon oxynitridefilm was formed by a plasma CVD method under the conditions where thesilicon wafer was held at a temperature of 220° C., silane at a flowrate of 50 sccm and dinitrogen monoxide at a flow rate of 2000 sccm wereused as a source gas, the pressure in the treatment chamber was 20 Pa,and a high-frequency power of 100 W at 13.56 MHz (1.6×10⁻² W/cm² as thepower density) was supplied to parallel-plate electrodes. Note that theflow ratio of dinitrogen monoxide to silane was 40.

<Sample A2>

For Sample A2, instead of the oxide insulating film of Sample A1, anoxide insulating film was formed under the following conditions.

In Sample A2, as the oxide insulating film, a 400-nm-thick siliconoxynitride film was formed by a plasma CVD method under the conditionswhere the silicon wafer was held at a temperature of 220° C., silane ata flow rate of 30 sccm and dinitrogen monoxide at a flow rate of 4000sccm were used as a source gas, the pressure in the treatment chamberwas 40 Pa, and a high-frequency power of 150 W at 13.56 MHz (8.0×10⁻²W/cm² as the power density) was supplied to parallel-plate electrodes.Note that the flow ratio of dinitrogen monoxide to silane was 133.

<Sample A3>

For Sample A3, instead of the oxide insulating film of Sample A1, anoxide insulating film was formed under the following conditions.

In Sample A3, as the oxide insulating film, a 100-nm-thick silicon oxidefilm was formed by a sputtering method under the conditions where thesilicon wafer was held at a temperature of 100° C., a silicon target wasused, oxygen at a flow rate of 50 sccm was used as a sputtering gas, thepressure in the treatment chamber was 0.5 Pa, and a high-frequency powerof 6 kW was supplied to parallel-plate electrodes.

<TDS Analysis>

TDS analyses were performed on Samples A1 to A3. In each sample, a stageon which the sample is mounted is heated at higher than or equal to 55°C. and lower than or equal to 997° C. The amounts of nitrogen monoxide(M/z=30), dinitrogen monoxide (M/z=44), nitrogen dioxide (M/z=46), andnitrogen (M/z=28) released from Samples A1 to A3 are shown in FIGS. 41A,41B, 41C and 42, respectively.

In FIGS. 41A to 41C and FIG. 42, the horizontal axis indicates thetemperature of the samples; here, the temperature range is from 50° C.to 650° C. inclusive. The upper limit temperature of an analysisapparatus used in this example is approximately 650° C. The verticalaxis indicates intensity proportional to the amount of each of thereleased gases. The total number of the molecules released to theoutside corresponds to the integral value of the peak. Thus, with thedegree of the peak intensity, the total number of the moleculescontained in the oxide insulating film can be evaluated.

In FIGS. 41A to 41C and FIG. 42, the bold solid line, the thin solidline, and the dashed line indicate the measurement results of SamplesA1, A2, and A3, respectively.

As shown in FIG. 41A to 41C and FIG. 42, peaks of M/z=30, M/z=44,M/z=46, and M/z=28 were observed in Sample A2 but were not observed inSamples A1 and A3. Note that the peak of Sample A1 observed at atemperature range of 150° C. to 200° C. in FIG. 41A is probablyattributed to release of a gas other than nitrogen monoxide. The peak ofSample A1 in FIG. 41B is probably attributed to release of a gas otherthan dinitrogen monoxide.

Next, FIG. 43 shows the amounts of nitrogen monoxide (M/z=30),dinitrogen monoxide (M/z=44), nitrogen dioxide (M/z=46), and nitrogen(M/z=28) released from Samples A1 and A2 calculated from integratedvalues of the peaks of the curves in FIGS. 41A to 41C and FIG. 42.

As shown in FIG. 43, the amounts of nitrogen monoxide, dinitrogenmonoxide, nitrogen dioxide, and nitrogen released from Sample A1 aresmaller than those from Sample A2 and are each lower than or equal tothe lower limit of detection; that is, the release of each gas is notdetected.

The above results show that, when the flow ratio of dinitrogen monoxideto silane in a source gas is small, the released amounts of nitrogenmonoxide, dinitrogen monoxide, nitrogen dioxide, and nitrogen arereduced.

Example 2

The amounts of hydrogen, carbon, nitrogen, and fluorine contained in theoxide insulating films of Samples A1 and A2 fabricated in Example 1 weremeasured by SIMS, and the results are described in this example.

In this example, silicon wafers were used as substrates of Samples A1and A2.

<SIMS Analysis>

SIMS analysis was performed on Samples A1 to A3. The concentration ofeach of hydrogen, carbon, nitrogen, and fluorine in each sample wasmeasured, from the surface of the oxide insulating film (SiON) towardthe silicon wafer (Si). FIGS. 44A and 44B show the measurement resultsof Samples A1 and A2, respectively.

In FIGS. 44A and 44B, the horizontal axis indicates a distance from thesurface of the oxide insulating film in the film thickness direction,and the vertical axis indicates the concentration of each element.Furthermore, in FIGS. 44A and 44B, the dashed, thin solid, bold solid,and dot-dashed lines indicate the concentrations of hydrogen, carbon,nitrogen, and fluorine, respectively. Note that Si and SiON indicateareas of the silicon wafer and the oxide insulating film, respectively.

In the oxide insulating film of Sample A1, the hydrogen concentration ishigher than or equal to 2×10²¹ atoms/cm³ and lower than or equal to5×10²¹ atoms/cm³; the nitrogen concentration is higher than or equal to6×10²⁰ atoms/cm³ and lower than or equal to 3×10²¹ atoms/cm³; the carbonconcentration gradually decreases from the surface toward the siliconwafer, and is higher than or equal to 4×10¹⁷ atoms/cm³ and lower than orequal to 5×10²⁰ atoms/cm³; and the fluorine concentration is higher thanor equal to 6×10¹⁸ atoms/cm³ and lower than or equal to 9×10¹⁸atoms/cm³.

In the oxide insulating film of Sample A2, the hydrogen concentration ishigher than or equal to 1×10²¹ atoms/cm³ and lower than or equal to3×10²¹ atoms/cm³; the nitrogen concentration is higher than or equal to1×10²⁰ atoms/cm³ and lower than or equal to 4×10²⁰ atoms/cm³; the carbonconcentration gradually decreases from the surface toward the siliconwafer, and is higher than lower than or equal to the lower limit ofdetection and lower than or equal to 6×10¹⁹ atoms/cm³; and the fluorineconcentration is higher than or equal to 7×10¹⁸ atoms/cm³ and lower thanor equal to 2×10¹⁸ atoms/cm³.

As shown in FIGS. 44A and 44B, the nitrogen concentration is higher inSample A1 than in Sample A2. This is probably because the oxideinsulating film of Sample A1 contains a lot of NH and NH₃ that do notbecome carrier traps.

In the case where the nitrogen concentration of the oxide insulatingfilm is lower than or equal to 6×10²⁰ atoms/cm³, the spin density islower than or equal to 1×10¹⁸ spins/cm³, and the oxide insulating filmhas a reduced number of defects caused by NO_(N).

Example 3

In this example, the number of defects in the oxide insulating film isdescribed using the measurement results of ESR.

<Fabrication Methods 1 of Samples>

Fabrication methods of Samples B1 to B3 of this example are describedbelow.

<Sample B1>

A 35-nm-thick oxide semiconductor film was formed over a quartzsubstrate by a sputtering method. The oxide semiconductor film wasformed in such a manner that a sputtering target of In:Ga:Zn=1:1:1(atomic ratio) was used, a sputtering gas in which the flow ratio ofargon and oxygen was 1:1 was supplied into a treatment chamber of asputtering apparatus, the pressure in the treatment chamber was adjustedto 0.6 Pa, and a direct-current power of 2.5 kW was supplied. Note thatthe oxide semiconductor film was formed at a substrate temperature of170° C.

Next, heat treatment was performed at 450° C. in a nitrogen atmospherefor one hour, and after that, another heat treatment was performed at450° C. in a mixed gas of nitrogen and oxygen for one hour.

Next, a first oxide insulating film and a second oxide insulating filmwere formed over the oxide semiconductor film.

The first oxide insulating film was formed to a thickness of 50 nm by aplasma CVD method under the following conditions: silane with a flowrate of 50 sccm and dinitrogen monoxide with a flow rate of 2000 sccmwere used as a source gas; the pressure in the treatment chamber was 20Pa; the substrate temperature was 220° C.; and a high-frequency power of100 W was supplied to parallel-plate electrodes.

The second oxide insulating film was formed to a thickness of 400 nm bya plasma CVD method under the following conditions: silane at a flowrate of 160 sccm and dinitrogen monoxide at a flow rate of 4000 sccmwere used as a source gas, the pressure in the treatment chamber was 200Pa, the substrate temperature was 220° C., and a high-frequency power of1500 W was supplied to parallel-plate electrodes. Under the aboveconditions, it is possible to form a silicon oxynitride film containingoxygen at a higher proportion than oxygen in the stoichiometriccomposition and from which part of oxygen is released by heating.

Next, by heat treatment, water, nitrogen, hydrogen, and the like werereleased from the first oxide insulating film and the second oxideinsulating film and part of oxygen contained in the second oxideinsulating film was supplied to the oxide semiconductor film. Here, theheat treatment was performed at 350° C. in a mixed atmosphere ofnitrogen and oxygen for one hour.

Through the above process, Sample B1 of this example was fabricated.

<Sample B2>

Sample B2, which was used for comparison, was fabricated under the sameconditions as those of Sample B1 except for the formation pressure ofthe first oxide insulating film.

In Sample B2, the first oxide insulating film was formed by a plasma CVDmethod under the following conditions: silane with a flow rate of 50sccm and dinitrogen monoxide with a flow rate of 2000 sccm were used asa source gas; the pressure in the treatment chamber was 100 Pa; thesubstrate temperature was 220° C.; and a high-frequency power of 100 Wwas supplied to parallel-plate electrodes.

<Sample B3>

Sample B3, which was used for comparison, was fabricated under the sameconditions as those of Sample B1 except for the formation pressure ofthe first oxide insulating film.

In Sample B3, the first oxide insulating film was formed by a plasma CVDmethod under the following conditions: silane at a flow rate of 50 sccmand dinitrogen monoxide at a flow rate of 2000 sccm were used as asource gas; the pressure in the treatment chamber was 200 Pa; thesubstrate temperature was 220° C.; and a high-frequency power of 100 Wwas supplied to parallel-plate electrodes.

<ESR Measurement>

Next, Samples B1 to B3 were measured by ESR measurement. Here, the ESRmeasurement was performed under the following conditions. Themeasurement temperature was −170° C., the high-frequency power (power ofmicrowaves) of 8.92 GHz was 1 mW, and the direction of a magnetic fieldwas parallel to a surface of each sample. The lower limit of detectionof the spin density of a signal attributed to NO is 4.7×10¹⁵ spins/cm³.This means that when the number of spins is small, the number of defectsis small in the film.

The spin densities of the signals attributed to NO of Samples B1 to B3are shown in FIGS. 45A to 45C, respectively. Note that shown here is thespin densities obtained by converting the number of measured spins intothat per unit volume.

As shown in FIGS. 45A to 45C, in Samples B1 to B3, a first signal thatappears at a g-factor of greater than or equal to 2.037 and smaller thanor equal to 2.039, a second signal that appears at a g-factor of greaterthan or equal to 2.001 and smaller than or equal to 2.003, and a thirdsignal that appears at a g-factor of greater than or equal to 1.964 andsmaller than or equal to 1.966 are observed. These three signals are dueto NO and represent splits of a hyperfine structure arising from theinteraction between an electron spin and the nuclear spin of a nitrogenatom. The signals attributed to NO have anisotropic spin species andthus the waveform is asymmetrical.

The spin density of the signals attributed to NO is higher in Samples B2and B3 than in Sample B1, and thus the oxide insulating films of SamplesB2 and B3 have a large number of defects. In FIGS. 45A to 45C, the spindensity of the signals attributed to NO in Sample B1 is the smallest.Thus, it is shown that when the first oxide insulating film to be incontact with the oxide semiconductor film is formed in high vacuum, theoxide insulating film having a reduced number of defects is formed.

<Fabrication Methods 2 of Samples>

Next, Samples B4 and B5 were fabricated: the formation pressure of thefirst oxide insulating film was fixed at the pressure of Sample B1,which obtained excellent results in the ESR measurement, and the flowratio of the deposition gas was changed. The number of defects ofSamples B4 and B5 were measured. The fabrication methods of Samples B4and B5 are shown below.

<Sample B4>

Sample B4, which was used for comparison, was fabricated under the sameconditions as those of Sample B1 except for the flow ratio of thedeposition gas for the first oxide insulating film.

In Sample B4, the first oxide insulating film was formed by a plasma CVDmethod under the following conditions: silane at a flow rate of 20 sccmand dinitrogen monoxide at a flow rate of 2000 sccm were used as asource gas; the pressure in the treatment chamber was 100 Pa; thesubstrate temperature was 220° C.; and a high-frequency power of 100 Wwas supplied to parallel-plate electrodes. In other words, when the flowratio of silane was 1, the flow ratio of dinitrogen monoxide was 100.

<Sample B5>

Sample B5, which was used for comparison, was fabricated under the sameconditions as those of Sample B1 except for the flow ratio of thedeposition gas for the first oxide insulating film.

In Sample B5, the first oxide insulating film was formed by a plasma CVDmethod under the following conditions: silane at a flow rate of 100 sccmand dinitrogen monoxide at a flow rate of 2000 sccm were used as asource gas; the pressure in the treatment chamber was 200 Pa; thesubstrate temperature was 220° C.; and a high-frequency power of 100 Wwas supplied to parallel-plate electrodes. In other words, when the flowrate of silane was 1, the flow rate of dinitrogen monoxide was 20.

<ESR Measurement>

Samples B1, B4, and B5 were measured by ESR measurement. FIGS. 46A, 46B,and 46C show the ESR measurement results of Samples B4, B1, and B5,respectively. The conditions of the ESR measurement were the same asthose of ESR measurement performed on Samples B1, B2, and B3.

As shown in FIGS. 46A and 46B, the spin densities of signals attributedto NO_(x) are higher in Sample B4, which was used for comparison, thanin Sample B1, and thus oxide insulating film of Samples B4 has a largenumber of defects. As shown in FIG. 46C, in Sample B5, which was usedfor comparison, the spin densities of signals attributed to NO_(x) arelower than or equal to the lower limit of detection, and a signalattributed to V_(o)H that appears at a g (g-factor) of 1.93 is observed.

Example 4

In this example, examination results of the I_(d)−V_(g) characteristicsand the reliability of fabricated transistors are described.

<Fabrication Methods 1 of Samples>

As Samples C1 to C3 of this example, transistors having the samestructure as that of the transistor 10 a in FIGS. 3A and 3B described inEmbodiment 1 were fabricated.

<Sample C1>

First, a glass substrate was used as the substrate 11, and the gateelectrode 13 was formed over the substrate 11.

The gate electrode 13 was formed in the following manner: a 100-nm-thicktungsten film was formed by a sputtering method, a mask was formed overthe tungsten film by a photolithography process, and the tungsten filmwas partly etched using the mask.

Next, the gate insulating film 15 was formed over the gate electrode 13.

As the gate insulating film 15, a stack including a 400-nm-thick siliconnitride film and a 50-nm-thick silicon oxynitride film was used.

Note that the silicon nitride film was formed to have a three-layerstructure of a first silicon nitride film, a second silicon nitridefilm, and a third silicon nitride film.

The first silicon nitride film was formed to a thickness of 50 nm underthe following conditions: silane at a flow rate of 200 sccm, nitrogen ata flow rate of 2000 sccm, and an ammonia gas at a flow rate of 100 sccmwere supplied to a treatment chamber of a plasma CVD apparatus as asource gas; the pressure in the treatment chamber was controlled to 100Pa; and power of 2000 W was supplied with the use of a 27.12 MHzhigh-frequency power source.

The second silicon nitride film was formed to a thickness of 300 nmunder the following conditions: silane at a flow rate of 200 sccm,nitrogen at a flow rate of 2000 sccm, and an ammonia gas at a flow rateof 2000 sccm were supplied to the treatment chamber of the plasma CVDapparatus as a source gas; the pressure in the treatment chamber wascontrolled to 100 Pa; and power of 2000 W was supplied with the use of a27.12 MHz high-frequency power source.

The third silicon nitride film was formed to a thickness of 50 nm underthe following conditions: silane at a flow rate of 200 sccm and nitrogenat a flow rate of 5000 sccm were supplied to the treatment chamber ofthe plasma CVD apparatus as a source gas; the pressure in the treatmentchamber was controlled to 100 Pa; and power of 2000 W was supplied withthe use of a 27.12 MHz high-frequency power source. Note that the firstsilicon nitride film, the second silicon nitride film, and the thirdsilicon nitride film were each formed at a substrate temperature of 350°C.

The silicon oxynitride film was formed under the following conditions:silane at a flow rate of 20 sccm and dinitrogen monoxide at a flow rateof 3000 sccm were supplied to the treatment chamber of the plasma CVDapparatus as a source gas; the pressure in the treatment chamber wascontrolled to 40 Pa; and power of 100 W was supplied with the use of a27.12 MHz high-frequency power source. Note that the silicon oxynitridefilm was formed at a substrate temperature of 350° C.

Next, the oxide semiconductor film 17 was formed to overlap with thegate electrode 13 with the gate insulating film 15 positionedtherebetween.

Here, a 35-nm-thick oxide semiconductor film was formed over the gateinsulating film 15 by a sputtering method, a mask was formed over theoxide semiconductor film by a photolithography process, and part of theoxide semiconductor film was etched with the use of the mask, wherebythe oxide semiconductor film 17 (S2-IGZO in FIG. 47) was formed.

The oxide semiconductor film 17 was formed under the followingconditions: an In—Ga—Zn oxide sputtering target containing In, Ga, andZn at an atomic ratio of 1:1:1 was used; oxygen at a flow proportion of50% was supplied as a sputtering gas into a treatment chamber of asputtering apparatus; the pressure in the treatment chamber wascontrolled to 0.6 Pa; and direct-current power of 2.5 kW was supplied.Note that the oxide semiconductor film was formed at a substratetemperature of 170° C.

Next, heat treatment was performed at 450° C. in a nitrogen atmospherefor one hour, and after that, another heat treatment was performed in amixed gas of nitrogen and oxygen at 450° C. for one hour.

Next, the pair of electrodes 19 and 20 in contact with the oxidesemiconductor film 17 were formed.

First, a conductive film was formed over the gate insulating film andthe oxide semiconductor film. As the conductive film, a 400-nm-thickaluminum film was formed over a 50-nm-thick tungsten film, and a100-nm-thick titanium film was formed over the aluminum film. Then, amask was formed over the conductive film by a photolithography process,and the conductive film was partly etched using the mask. Through theabove steps, the pair of electrodes 19 and 20 were formed.

Next, the substrate was transferred to a treatment chamber in a reducedpressure and heated at 220° C. Then, the oxide semiconductor film 17 wasexposed to oxygen plasma that was generated in a dinitrogen monoxideatmosphere by supply of a high-frequency power of 150 W to an upperelectrode in the treatment chamber with the use of a 27.12 MHzhigh-frequency power source.

After that, the protective film 21 was formed over the oxidesemiconductor film 17 and the pair of electrodes 19 and 20. In thiscase, the protective film 21 was formed to have a three-layer structureof a first oxide insulating film (P1-SiON in FIG. 47), a second oxideinsulating film (P2-SiON in FIG. 47), and a nitride insulating film.

The 50-nm-thick first oxide insulating film was formed by a plasma CVDmethod under the following conditions: silane at a flow rate of 50 sccmand dinitrogen monoxide at a flow rate of 2000 sccm were used as asource gas; the pressure in the treatment chamber was 20 Pa; thesubstrate temperature was 220° C.; and a high-frequency power of 100 Wwas supplied to parallel-plate electrodes.

The formation conditions of the first oxide insulating film of Sample C1is the same as those of the first oxide insulating film of Sample B1described in Example 3.

The 400-nm-thick second oxide insulating film was formed by a plasma CVDmethod under the following conditions: silane at a flow rate of 160 sccmand dinitrogen monoxide at a flow rate of 4000 sccm were used as asource gas, the pressure in the treatment chamber was 200 Pa, thesubstrate temperature was 220° C., and a high-frequency power of 1500 Wwas supplied to parallel-plate electrodes. Under the above conditions,it is possible to form a silicon oxynitride film that contains oxygen ata higher proportion than oxygen in the stoichiometric composition sothat part of oxygen is released by heating.

Next, heat treatment was performed to release water, nitrogen, hydrogen,and the like from the first oxide insulating film and the second oxideinsulating film and to supply part of oxygen contained in the secondoxide insulating film into the oxide semiconductor film. Here, the heattreatment was performed at 350° C. in a mixed atmosphere of nitrogen andoxygen for one hour.

Then, a 100-nm-thick nitride insulating film was formed over the secondoxide insulating film. The nitride insulating film was formed by aplasma CVD method under the following conditions: silane at a flow rateof 50 sccm, nitrogen at a flow rate of 5000 sccm, and an ammonia gas ata flow rate of 100 sccm were used as a source gas; the pressure in thetreatment chamber was 100 Pa; the substrate temperature was 350° C.; anda high-frequency power of 1000 W was supplied to parallel-plateelectrodes.

Next, a planarization film was formed (not illustrated) over theprotective film 21. Here, the protective film 21 was coated with acomposition, and exposure and development were performed, so that aplanarization film having an opening through which the pair ofelectrodes are partly exposed was formed. Note that as the planarizationfilm, a 1,5-μm-thick acrylic resin was formed. Then, heat treatment wasperformed. The heat treatment was performed in a nitrogen atmosphere at250° C. for one hour.

Then, an opening was formed in part of the protective film 21 so thatthe opening reached one of the pair of electrodes 19 and 20. The openingwas formed by etching part of the protective film 21 using theplanarization film as a mask.

Next, a pixel electrode was formed over the planarization film so thatthe pixel electrode was electrically connected to one of the pair ofelectrodes 19 and 20 through the opening formed in parts of theprotective film 21 and the planarization film.

Here, as the pixel electrode, a conductive film of an indium oxide-tinoxide compound (ITO-SiO₂) containing silicon oxide was formed by asputtering method. Note that the composition of a target used forforming the conductive film was In₂O₃:SnO₂:SiO₂=85:10:5 [wt %]. Afterthat, heat treatment was performed at 250° C. in a nitrogen atmospherefor one hour.

Through the above process, Sample C1 of this example was fabricated.

<Sample C2>

Sample C2 was fabricated under the same conditions as those of Sample C1except for the formation pressure of the first oxide insulating film.

In Sample C2, the first oxide insulating film was formed by a plasma CVDmethod under the following conditions: silane at a flow rate of 50 sccmand dinitrogen monoxide at a flow rate of 2000 sccm were used as asource gas; the pressure in the treatment chamber was 100 Pa; thesubstrate temperature was 220° C.; and a high-frequency power of 100 Wwas supplied to parallel-plate electrodes.

The formation conditions of the first oxide insulating film of Sample C2is the same as those of the first oxide insulating film of Sample B2described in Example 3.

<Sample C3>

Sample C3, which was used for comparison, was fabricated under the sameconditions as those of Sample C1 except for the formation pressure ofthe first oxide insulating film.

In Sample C3, the first oxide insulating film was formed by a plasma CVDmethod under the following conditions: silane with a flow rate of 50sccm and dinitrogen monoxide with a flow rate of 2000 sccm were used asa source gas; the pressure in the treatment chamber was 200 Pa; thesubstrate temperature was 220° C.; and a high-frequency power of 100 Wwas supplied to parallel-plate electrodes.

The formation conditions of the first oxide insulating film of Sample C3is the same as those of the first oxide insulating film of Sample B3described in Example 3.

<I_(d)-V_(g) Characteristics>

Next, initial I_(d)-V_(g) characteristics of the transistors included inSamples C1 to C3 were measured. Here, changes in characteristics ofcurrent flowing between a source and a drain (hereinafter referred to asdrain current: I_(d)), that is, I_(d)-V_(g) characteristics weremeasured under the following conditions: the substrate temperature was25° C., the potential difference between the source and the drain(hereinafter referred to as drain voltage: V_(d)) was 1 V or 10 V, andthe potential difference between the source and the gate electrodes(hereinafter referred to as gate voltage: V_(g)) was changed from −15 Vto 15 V.

FIG. 47 shows I_(d)-V_(g) characteristics of Samples C1 to C3. FIG. 47shows the results of transistors each having a channel length L of 6 μmand a channel width W of 50 μm. In FIG. 47, the horizontal axis, thefirst vertical axis, and the second vertical axis represent gate voltageV_(g), drain current I_(d), and field-effect mobility, respectively.Here, to show field-effect mobility in a saturation region, field-effectmobility calculated when V_(d)=10 V is shown.

As shown in FIG. 47, Samples C1 and C2 have excellent initialI_(d)-V_(g) characteristics. In contrast, Sample C3, which was used forcomparison and in which the formation pressure of the first oxideinsulating film was 200 Pa, has variations in I_(d)-V_(g)characteristics.

<Gate BT Stress Test>

Next, a gate BT stress test (GBT) and a gate BT photostress test (PGBT)were performed on Samples C1 to C3.

First, a gate BT stress test and a gate BT photostress test wereperformed.

A measurement method of the gate BT stress test is described. First,initial I_(d)-V_(g) characteristics of the transistor were measured asdescribed above.

Next, the substrate temperature was kept constant at a given temperature(hereinafter referred to as stress temperature), the pair of electrodesserving as a source electrode and a drain electrode of the transistorwas set at the same potential, and the gate electrode was supplied witha potential different from that of the pair of electrodes for a certainperiod of time (hereinafter referred to as stress time). Next, thesubstrate temperature was set as appropriate, and the electricalcharacteristics of the transistor were measured. As a result, adifference in threshold voltage and a difference in shift value betweenbefore and after the gate BT stress test can be obtained as the amountof change in the electrical characteristics.

Note that a stress test where negative voltage is applied to a gateelectrode is called negative gate BT stress test (dark negative stress);whereas a stress test where positive voltage is applied is calledpositive gate BT stress test (dark positive stress). Note that a stresstest where negative voltage is applied to a gate electrode while lightemission is performed is called negative gate BT photostress test(negative photostress); whereas a stress test where positive voltage isapplied while light emission is performed is called positive gate BTphotostress test (positive photostress).

Here, the gate BT stress conditions were as follows: stress temperaturewas 60° C., stress time was 3600 seconds, −30 V or +30 V was applied tothe gate electrode, and 0 V was applied to the pair of electrodesserving as the source electrode and the drain electrode. The electricfield intensity applied to the gate insulating film was 0.66 MV/cm.

Under the same conditions as those of the above gate BT stress test, thegate BT photostress test was performed where the transistor wasirradiated with white light with 10000 1× using an LED. Note that theV_(g)-I_(d) characteristics of the transistor were measured at atemperature of 60° C. after each of the BT stress tests.

FIG. 48 shows a difference between threshold voltage in the initialcharacteristics and threshold voltage after the BT stress test (i.e.,the amount of change in threshold voltage (ΔV_(th))) and a difference inshift value (i.e., the amount of change in the shift value (AShift)) ofrespective transistors included in Samples C1 to C3.

Here, a threshold voltage and a shift value in this specification aredescribed. Threshold voltage V_(th) is defined as, in the I_(d)-V_(g)curve where the horizontal axis represents gate voltage V_(g) [V] andthe vertical axis represents the square root of drain current I_(d)(I_(d) ^(1/2)) [A^(1/2)], gate voltage at the intersection point of theline of I_(d) ^(1/2)=0 (V_(g) axis) and the tangent to the curve at apoint where the slope of the curve is the steepest. Note that here, thethreshold voltage is calculated with a drain voltage V_(d) of 10 V.

Furthermore, shift value Shift in this specification is defined as, inthe I_(d)-V_(g) curve where the horizontal axis represents the gatevoltage V_(g) [V] and the vertical axis represents the logarithm of thedrain current I_(d) [A], gate voltage at the intersection point of theline of I_(d)=1.0×10⁻¹² [A] and the tangent to the curve at a pointwhere the slope of the curve is the steepest. Note that here, the shiftvalue is calculated with a drain voltage V_(d) of 10 V.

From FIG. 48, the amount of change in the threshold voltage and theamount of change in the shift value were smaller in Samples C1 and C2than in Sample C3, which was used for comparison. In particular, inSample C1, the amount of change in the threshold voltage and the amountof change in the shift value were small in the positive gate BTphotostress test and the negative gate BT photostress test.

<Fabrication Methods 2 of Samples>

Next, Samples C4 and C5 were fabricated: the formation pressure of thefirst oxide insulating film was fixed at the pressure of Sample C1,which obtained the excellent I_(d)-V_(g) characteristics and excellentresults in the gate BT stress test, and the flow ratio of the depositiongas was changed. The I_(d)-V_(g) characteristics and reliability ofSamples C4 and C5 were measured. The fabrication methods of Samples C4and C5 are shown below.

<Sample C4>

Sample C4 was fabricated under the same conditions as those of Sample C1except for the flow ratio of the deposition gas for the first oxideinsulating film.

In Sample C4, the first oxide insulating film was formed by a plasma CVDmethod under the following conditions: silane at a flow rate of 20 sccmand dinitrogen monoxide at a flow rate of 2000 sccm were used as asource gas; the pressure in the treatment chamber was 100 Pa; thesubstrate temperature was 220° C.; and a high-frequency power of 100 Wwas supplied to parallel-plate electrodes. In other words, when the flowrate of silane was 1, the flow rate of dinitrogen monoxide was 100.

The formation conditions of the first oxide insulating film of Sample C4is the same as those of the first oxide insulating film of Sample B4described in Example 3.

<Sample C5>

Sample C5, which was used for comparison, was fabricated under the sameconditions as those of Sample C1 except for the flow ratio of thedeposition gas for the first oxide insulating film.

In Sample C5, the first oxide insulating film was formed by a plasma CVDmethod under the following conditions: silane at a flow rate of 100 sccmand dinitrogen monoxide at a flow rate of 2000 sccm were used as asource gas; the pressure in the treatment chamber was 200 Pa; thesubstrate temperature was 220° C.; and a high-frequency power of 100 Wwas supplied to parallel-plate electrodes. In other words, when the flowrate of silane was 1, the flow rate of dinitrogen monoxide was 20.

The formation conditions of the first oxide insulating film of Sample C5is the same as those of the first oxide insulating film of Sample B5described in Example 3.

<I_(d)-V_(g) Characteristics>

Next, initial I_(d)-V_(g) characteristics of the transistors included inSamples C1, C4, and C5 were measured. Here, changes in drain currentI_(d), that is, I_(d)-V_(g) characteristics were measured under thefollowing conditions: the substrate temperature was 25° C., the drainvoltage V_(d) was 1 V or 10 V, and the gate voltage V_(g) was changedfrom −15 V to 15 V.

FIG. 49 shows I_(d)-V_(g) characteristics of Samples C1, C4, and C5.FIG. 49 shows the results of transistors having a channel length L of 6μm and a channel width W of 50 μm. In FIG. 49, the horizontal axis, thefirst vertical axis, and the second vertical axis represent gate voltageV_(g), drain current I_(d), and field-effect mobility, respectively.Here, to show field-effect mobility in a saturation region, field-effectmobility calculated when V_(d)=10 V is shown.

As shown in FIG. 49, Samples C1 and C4 have excellent initialI_(d)-V_(g) characteristics. In contrast, in Sample C5, which was usedfor comparison, the on-off ratio of the drain current is not obtained;thus, the transistor characteristics are not obtained. In considerationof the results of Sample B5 described in Example 3, this is probablybecause the oxide semiconductor film contains a large number of oxygenvacancies.

<Gate BT Stress Test>

Next, a gate BT stress test and a gate BT photostress test wereperformed on Samples C1, C4, and C5.

Specifically, a positive gate BT stress test (dark positive stress), anegative gate BT stress test (dark negative stress), a positive gate BTphotostress test (positive photostress), and a negative gate BTphotostress test (negative photostress) were performed. FIG. 50 showsthe difference between the initial threshold voltage and the thresholdvoltage after the gate BT stress test (i.e., the amount of change in thethreshold voltage (ΔV_(th))) and the difference between the initialshift value and the shift value after the gate BT stress test (i.e., theamount of change in the shift value (AShift)) of transistors of SamplesC1, C4, and C5.

As shown in FIG. 50, the amount of change in the threshold voltage andthe amount of change in the shift value were greater in Sample C4, whichwas used for comparison and in which the flow ratio of dinitrogenmonoxide to silane was 100 in forming the first oxide insulating film,than in Sample C1 of one embodiment of the present invention in whichthe flow ratio of dinitrogen monoxide to silane was 40.

According to Example 3 and this example, the oxide insulating film incontact with the oxide semiconductor film in Sample C1 has a small spindensity, in other words, a small number of defects, and thus the amountof change in the threshold voltage and the amount of change in the shiftvalue of the transistor are small.

<Fabrication Methods 3 of Samples>

Next, Samples C6 and C7 were fabricated by changing at least one of theflow ratio of the deposition gas, the pressure, and the formationtemperature. The I_(d)-V_(g) characteristics and reliability of SamplesC6 and C7 were measured. The fabrication methods of Samples C6 and C7are shown below.

<Sample C6>

Sample C6, which was used for comparison, was fabricated under the sameconditions as those of Sample C1 except for the flow ratio of thedeposition gas for the first oxide insulating film.

In Sample C6, the first oxide insulating film was formed by a plasma CVDmethod under the following conditions: silane at a flow rate of 30 sccmand dinitrogen monoxide at a flow rate of 4000 sccm were used as asource gas; the pressure in the treatment chamber was 40 Pa; thesubstrate temperature was 220° C.; and a high-frequency power of 100 Wwas supplied to parallel-plate electrodes. In other words, when the flowrate of silane was 1, the flow rate of dinitrogen monoxide was 133.

<Sample C7>

Sample C7, which was used for comparison, was fabricated under the sameconditions as those of Sample C1 except for the flow ratio of thedeposition gas for the first oxide insulating film.

In Sample C7, the first oxide insulating film was formed by a plasma CVDmethod under the following conditions: silane at a flow rate of 20 sccmand dinitrogen monoxide at a flow rate of 3000 sccm were used as asource gas; the pressure in the treatment chamber was 200 Pa; thesubstrate temperature was 350° C.; and a high-frequency power of 100 Wwas supplied to parallel-plate electrodes. In other words, when the flowrate of silane was 1, the flow rate of dinitrogen monoxide was 150.

<Gate BT Stress Test>

Next, a gate BT stress test and a gate BT photostress test wereperformed on Samples C6 and C7. Here, the description of the testresults is omitted.

A sample having a structure similar to that described in Example 3 wasfabricated using the same conditions as those of the oxide semiconductorfilm, the first oxide insulating film, and the second oxide insulatingfilm of Sample C6. This sample is referred to as Sample B6. A samplehaving a structure described in Example 3 was fabricated using the sameconditions as those the oxide semiconductor film, the first oxideinsulating film, and the second oxide insulating film of Sample C7. Thissample is referred to as Sample B7. ESR measurement was performed alsoin Samples B6 and B7, and the spin densityof signals attributed toNO_(x) was obtained. Here, the description of the measurement results ofESR is omitted.

<Amount of Change in Spin Density and Amount of Change in ThresholdVoltage of Oxide Insulating Film>

FIG. 51 shows the spin density of the samples obtained in Example 3 andthe amount of change in the threshold voltage of the samples obtained inExample 4. Here, the horizontal axis indicates the spin densities ofSamples B1, B2, B4, B6, and B7, and the vertical axis indicates theamount of change in the threshold voltage due to a negative gate BTstress test (dark negative stress) of Samples C1, C2, C4, C6, and C7.

FIG. 51 shows that when the spin density of each sample is small, theamount of change in threshold voltage is small. In Samples B1, B2, B4,B6, B7, C1, C2, C4, C6, and C7, the formation conditions of the oxidesemiconductor film and the second oxide insulating film were the same,and when the spin density of signals attributed to NO_(x) of the firstoxide insulating film is lower than, typically, 1×10¹⁸ spins/cm³, theamount of change in the threshold voltage was small.

Example 5

Described in this example is the diffusion of oxygen in an oxideinsulating film that is in contact with an oxide semiconductor film andhas a stacked-layer structure like the oxide insulating film describedin Modification example 1 in Embodiment 1. In this example, the oxygenconcentration was measured by SSDP-SIMS (SIMS measurement from thesubstrate side) to describe the diffusion of oxygen.

<Sample D1>

A method for fabricating Sample D1 is described.

First, a 100-nm-thick oxide semiconductor film (IGZO in FIGS. 52A and52B) was formed over a glass substrate (Glass in FIGS. 52A and 52B) by asputtering method using an In—Ga—Zn oxide sputtering target whereIn:Ga:Zn=1:1:1 (atomic ratio) and using oxygen and argon as sputteringgases.

Next, a first oxide insulating film (SiON in FIGS. 52A and 52B) and asecond oxide insulating film (SP-SiO_(x) in FIGS. 52A and 52B) wereformed over the oxide semiconductor film. As the second oxide insulatingfilm, a silicon oxide film containing oxygen at a higher proportion thanoxygen in the stoichiometric composition was formed.

Here, as the first oxide insulating film, a 30-nm-thick siliconoxynitride film was formed by a plasma CVD method under the followingconditions: silane at a flow rate of 30 sccm and dinitrogen monoxide ata flow rate of 4000 sccm were used as a source gas, the pressure in thetreatment chamber was 200 Pa, the substrate temperature was 220° C., anda high-frequency power of 150 W was supplied to parallel-plateelectrodes.

As the second oxide insulating film, a 100-nm-thick silicon oxide filmcontaining ¹⁸O was formed by a sputtering method in which a siliconwafer was placed in a treatment chamber of a sputtering apparatus, and¹⁸O (an isotope of ¹⁶O) with a flow rate of 300 sccm as a source gas wassupplied to the treatment chamber.

Through the above process, Sample D1 was fabricated.

<Sample D2>

A method for fabricating Sample D2 is described.

Sample D1 was heated at 350° C. in an atmosphere of a mixed gascontaining nitrogen and oxygen for one hour.

Through the above process, Sample D2 was fabricated.

<SIMS Analysis>

Next, the concentration profiles of ¹⁸O contained in the first oxideinsulating films SiON and the oxide semiconductor films IGZO of SamplesD1 and D2 were measured by SIMS. Here, the concentration of ¹⁸O wasmeasured from the glass substrate side to the second oxide insulatingfilm.

FIGS. 52A and 52B each show the concentration profiles of ¹⁸O that wereobtained by the SIMS measurement. The first oxide insulating film SiONwas quantified and the results are shown in FIG. 52A, and the oxidesemiconductor film IGZO was quantified and the results are shown in FIG.52B. In FIGS. 52A and 52B, thin solid line and the bold solid lineindicate the measurement results of Samples D1 and D2, respectively.

As shown in FIG. 52A, the concentration of ¹⁸O increases in the firstoxide insulating film SiON in Sample D2. As shown in FIG. 52B, theconcentration of ¹⁸O increases in the oxide semiconductor film IGZO onthe first oxide insulating film SiON side in Sample D2.

The above results indicate that oxygen is diffused by heat treatmentfrom the second oxide insulating film SP-SiO_(x) through the first oxideinsulating film SiON to the oxide semiconductor film IGZO.

Example 6

Described in this example is heat treatment and the number of oxygenvacancies in an oxide insulating film that is in contact with an oxidesemiconductor film and has a stacked-layer structure like the oxideinsulating film described in Modification example 1 in Embodiment 1. Inthis example, the number of oxygen vacancies in the oxide semiconductorfilm is described using the measurement results of ESR.

<Sample E1>

A method for fabricating Sample E1 is described.

First, a 35-nm-thick oxide semiconductor film was formed over a quartzsubstrate by a sputtering method using an In—Ga—Zn oxide sputteringtarget where In:Ga:Zn=1:1:1 (the ratio of the number of atoms) and usinga sputtering gas of oxygen and argon.

Next, heat treatment was performed at 450° C. in a nitrogen atmospherefor one hour, and then another heat treatment was performed at 450° C.in a mixed gas of nitrogen and oxygen for one hour.

Next, a first oxide insulating film and a second oxide insulating filmwere formed over the oxide semiconductor film. As the second oxideinsulating film, a silicon oxynitride film containing oxygen at a higherproportion than oxygen in the stoichiometric composition was formed.

Here, as the first oxide insulating film, a 50-nm-thick siliconoxynitride film was formed. The first oxide insulating film was formedby a plasma CVD method under the following conditions: silane at a flowrate of 30 sccm and dinitrogen monoxide at a flow rate of 4000 sccm wereused as a source gas, the pressure in the treatment chamber was 40 Pa,the substrate temperature was 220° C., and a high-frequency power of 150W was supplied to parallel-plate electrodes.

As the second oxide insulating film, a 400-nm-thick silicon oxynitridefilm was formed. The second oxide insulating film was formed by a plasmaCVD method under the following conditions: silane at a flow rate of 160sccm and dinitrogen monoxide at a flow rate of 4000 sccm were used as asource gas, the pressure in the treatment chamber was 200 Pa, thesubstrate temperature was 220° C., and a high-frequency power of 1500 Wwas supplied to parallel-plate electrodes.

Through the above process, Sample E1 was fabricated.

<Sample E2>

A method for forming Sample E2 is described.

Sample E1 was heated at 350° C. in an atmosphere of a mixed gascontaining nitrogen and oxygen for one hour.

Through the above process, Sample E2 was formed.

<ESR Measurement>

Next, Samples E1 and E2 were measured by ESR measurement. In the ESRmeasurement performed at a predetermined temperature, a value of amagnetic field (H₀) where a microwave is absorbed is used for anequation g=h v/βH₀; thus, a parameter “g-factor” can be obtained. Notethat the frequency of the microwave is denoted by v, and the Planckconstant and the Bohr magneton are denoted by, respectively, h and βthat are both constants.

Here, the ESR measurement was performed under the following conditions.Here, the ESR measurement was performed under the following conditions:the measurement temperature was room temperature (25° C.), thehigh-frequency power (power of microwaves) of 8.9 GHz was 20 mW, and thedirection of a magnetic field was parallel to a surface of each sample.Note that the lower limit of detection of the spin density of a signalattributed to V_(o)H in the IGZO film, which appeared at a g (g-factor)of 1.93, was 1×10¹⁷ spins/cm³.

FIGS. 53A and 53B show ESR spectra obtained by ESR measurement. FIGS.53A and 53B show ESR spectra of the oxide semiconductor films of SamplesE1 and E2, respectively.

As shown in FIG. 53A, in Sample E1, a signal attributed to V_(o)Happears at a g (g-factor) of 1.93. The number of spins absorbed at a g(g-factor) of 1.93 is 5.14×10¹⁸ spins/cm³. This means that the oxidesemiconductor film contains V_(o)H.

In contrast, as shown in FIG. 53B, in Sample E2, a signal attributed toV_(o)H appearing at a g (g-factor) of 1.93 is not observed.

The above results confirmed that V_(o)H in the oxide semiconductor filmcan be reduced by heat treatment. Furthermore, the results described inExample 5 reveal that oxygen contained in the oxide insulating filmcontaining oxygen at a higher proportion than oxygen in thestoichiometric composition is diffused to the oxide semiconductor filmby heat treatment. This means that when oxygen is diffused to the oxidesemiconductor film by heat treatment, V_(o)H in the oxide semiconductorfilm can be reduced.

Example 7

In this example, oxidizing power of plasma caused when an oxideinsulating film was exposed to plasma generated by using dinitrogenmonoxide or oxygen as an oxidizing gas is described.

First, a method of fabricating each sample is described.

A 100-nm-thick silicon oxynitride film was formed as an oxide insulatingfilm containing nitrogen over a quartz substrate. Then, the siliconoxynitride film was exposed to plasma that was generated in an oxidizinggas atmosphere. Conditions of the formation of the silicon oxynitridefilm and conditions of plasma treatment are described below.

The silicon oxynitride film was formed under the conditions as follows:the quartz substrate was placed in a treatment chamber of a plasma CVDapparatus; silane with a flow rate of 1 sccm and dinitrogen monoxidewith a flow rate of 800 sccm that were used as a source gas weresupplied to the treatment chamber; the pressure in the treatment chamberwas controlled to 40 Pa: and the power of 150 W was supplied with theuse of a 60 MHz high-frequency power source. Further, the temperature ofthe quartz substrate at the formation of the silicon oxynitride film was400° C. Note that the plasma CVD apparatus used in this example is aparallel plate plasma CVD apparatus in which the electrode area is 615cm², and the power per unit area (power density) into which the suppliedpower is converted is 0.24 W/cm².

Plasma was generated in such a manner that dinitrogen monoxide or oxygenwith a flow rate of 900 sccm was supplied to the treatment chamber, thepressure in the treatment chamber was controlled to 200 Pa, and power of900 W (1.46 W/cm²) was supplied with the use of a 60 MHz high-frequencypower source. Further, the temperature of the quartz substrate at thetime of plasma generation was 200° C. Here, a sample that was exposed toplasma generated in a dinitrogen monoxide atmosphere is referred to asSample F1. In addition, a sample that was exposed to plasma generated inan oxygen atmosphere is referred to as Sample F2.

Next, TDS analyses were performed on Samples F1 and F2.

The peaks of the curves shown in the results obtained from TDS analysesappear due to release of atoms or molecules contained in the analyzedsamples (in this example, Samples F1 and F2) to the outside. The totalamount of the atoms or molecules released to the outside corresponds tothe integral value of the peak. Thus, with the degree of the peakintensity, the number of the atoms or molecules contained in the siliconoxynitride film can be evaluated.

FIGS. 54A and 54B show the results of the TDS analyses on Samples F1 andF2. FIGS. 54A and 54B are each a graph showing the number of releasedoxygen molecules versus the substrate temperature.

FIGS. 54A and 54B demonstrate that the silicon oxynitride film that wasexposed to plasma generated in a dinitrogen monoxide atmosphere hashigher TDS intensity of oxygen molecules than the silicon oxynitridefilm that was exposed to plasma generated in an oxygen atmosphere. Asdescribed above, plasma generated in a dinitrogen monoxide atmospherehas stronger oxidizing power than plasma generated in an oxygenatmosphere and enables formation of a film containing excess oxygen,from which oxygen is released easily by heating.

Accordingly, in the case where an oxide insulating film is formed overan oxide semiconductor film by a plasma CVD method, a film containingexcess oxygen, from which oxygen can be released by heating, can beformed by using a deposition gas containing silicon and dinitrogenmonoxide as a source gas. Note that when dinitrogen monoxide is used asa source gas, nitrogen is contained in the oxide insulating film;therefore, an oxide insulating film containing nitrogen and excessoxygen can be obtained.

This application is based on Japanese Patent Application serial no.2013-173958 filed with Japan Patent Office on Aug. 23, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a gateinsulating film; an oxide semiconductor film; and a protective film,wherein the oxide semiconductor film is provided between the gateinsulating film and the protective film, wherein at least one of thegate insulating film and the protective film has a spin density obtainedby an electron spin resonance spectrum, and wherein the spin density ishigher than or equal to 1×10¹⁷ spins/cm³ and lower than 1×10¹⁸spins/cm³.
 2. The semiconductor device according to claim 1, wherein thespin density is attributed to nitrogen oxide.
 3. The semiconductordevice according to claim 2, wherein the nitrogen oxide is nitrogenmonoxide or nitrogen dioxide.
 4. The semiconductor device according toclaim 1, further comprising a gate electrode, wherein the gate electrodeis under the gate insulating film.
 5. The semiconductor device accordingto claim 1, further comprising a gate electrode, wherein the gateelectrode is over the gate insulating film.
 6. The semiconductor deviceaccording to claim 1, further comprising a pair of electrodes, whereinthe pair of electrodes are provided between the oxide semiconductor filmand the protective film.
 7. The semiconductor device according to claim1, further comprising a pair of electrodes, wherein the pair ofelectrodes are provided between the oxide semiconductor film and thegate insulating film.
 8. The semiconductor device according to claim 1,wherein the oxide semiconductor film comprises indium, gallium, andzinc.
 9. The semiconductor device according to claim 1, wherein theoxide semiconductor film comprises indium, tin, and zinc.
 10. Asemiconductor device comprising: a gate insulating film; an oxidesemiconductor film; and a protective film, wherein the oxidesemiconductor film is provided between the gate insulating film and theprotective film, and wherein at least one of the gate insulating filmand the protective film has a spin density obtained by an electron spinresonance spectrum of lower than 1×10¹⁸ spins/cm³, wherein the electronspin resonance spectrum comprises a first signal, a second signal, and athird signal, wherein a first signal appears at a g-factor of greaterthan or equal to 2.037 and smaller than or equal to 2.039, wherein asecond signal appears at a g-factor of greater than or equal to 2.001and smaller than or equal to 2.003, and wherein a third signal appearsat a g-factor of greater than or equal to 1.964 and smaller than orequal to 1.966.
 11. The semiconductor device according to claim 10,wherein a split width of the first and second signals and a split widthof the second and third signals are each 5 mT.
 12. The semiconductordevice according to claim 10, wherein the spin density is higher than orequal to 1×10¹⁷ spins/cm³ and lower than 1×10¹⁸ spins/cm³.
 13. Thesemiconductor device according to claim 10, wherein the spin density isattributed to nitrogen oxide.
 14. The semiconductor device according toclaim 13, wherein the nitrogen oxide is nitrogen monoxide or nitrogendioxide.
 15. The semiconductor device according to claim 10, furthercomprising a gate electrode, wherein the gate electrode is under thegate insulating film.
 16. The semiconductor device according to claim10, further comprising a gate electrode, wherein the gate electrode isover the gate insulating film.
 17. The semiconductor device according toclaim 10, further comprising a pair of electrodes, wherein the pair ofelectrodes are provided between the oxide semiconductor film and theprotective film.
 18. The semiconductor device according to claim 10,further comprising a pair of electrodes, wherein the pair of electrodesare provided between the oxide semiconductor film and the gateinsulating film.
 19. The semiconductor device according to claim 10,wherein the oxide semiconductor film comprises indium, gallium, andzinc.
 20. The semiconductor device according to claim 10, wherein theoxide semiconductor film comprises indium, tin, and zinc.